Dungeons & Dragons offers countless character design variations and mechanics that can feel truly infinite. Unfortunately for dungeon masters, this can lead to loopholes and unaccounted for exploits that give players far more power than they otherwise should have far too soon.

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These players can affect how the game plays out for the rest of the table, making encounters mundane and low-stakes, or completely removing the ability to play certain scenarios. As a Dungeon Master, there are a number of correct ways to deal with players using exploits that help keep your sessions intact.
8
Recycle magic items
There hasn't been a DM that has created a magic item for their campaign that hasn't ended up being way too overpowered. Now your players use it to destroy every encounter they would otherwise lose. Here, you can add arbitrary limitations to the item, such as daily charges, or even allow it to disintegrate, such as the Binding Wand.
If you don't want to completely change the rules, you can give the item a hidden curse that forces players to act in bad ways, forcing them to choose between staying good or being strong. If nothing else, a complete rebalancing of the item may fix these issues, despite admitting the flaws.
7
Restrict specific combinations
Multiclassing is a significant source of player exploits that allow them to travel at supersonic speeds and summon massive hordes of allies and creatures. In your campaign, since you are allowed to ban certain species and spells, you can ban several classes that you know are causing problems for the table.
This can even be retroactive, forcing a player using a multiclass exploit to choose one or the other, which can be disastrous for that player, but could save the game for everyone else. This is a better solution than banning a class or spell because it only removes niche options.
You can review character sheets before your first session to make sure they are filled out correctly and don't break any house rules.
6
Rebalance encounters
Instead of attacking the source of the exploit, you can rebalance your social and combat encounters to prevent the player from succeeding every time. The next time your Bard player uses an exploit to always successfully charm the merchant into giving away his supplies, give the NPC a magic item that makes him immune to the effect.

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If your fighter is capable of 16 attacks per turn due to complex interactions with Echo Knight and Action Surge, use monsters that reflect melee attack damage, such as an Azer. In any power-up competition between players and the DM, the DM wins every time.
5
Apply Consequences
One of the biggest problems with players using exploits in D&D is that it usually requires players to ignore the role-playing and storytelling aspects in order to focus on the details of the game mechanics. If your world reacts accordingly to these strange characters, it can be a way to punish unrealistic behavior.
Players able to destroy monsters with a massive hit might get an unwanted visit from the monster hunting guild who can't get work due to the carnage. News of strong fighters could attract the attention of legendary champions who want to test their skills and have a better chance of winning.
4
Use Antimagic
The easiest way to address player exploits is to use an in-game kill switch, such as an anti-magic field, that reduces players from throwing rocks and daggers. Players using exploits usually fail to pull off their only combo, leading to an unbalanced game, forcing the rest of the party to pick up their game.
This can signal to that player that they need to rebalance their characters to adapt to unique situations without relying on a single exploit or technique. For non-magical characters, using rust monsters or the Heat Metal spell works the same way, shutting down their primary resource.
3
Use their build against them
Whatever your players are capable of accomplishing in the world is also true for every other NPC they encounter. By using the same exploits against them, you can create encounters that show just how dangerous or magical the world around them can be.

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This is a popular tactic to use a rival party that mirrors the main party, such as the one created for Call of the Netherdeep. This shouldn't be a way to punish the player using exploits, but rather to show them that they won't be able to compete with M and shouldn't try.
One of the most common exploits seen in D&D veterans is using out-of-game knowledge of creatures' stats and spells to make decisions their character wouldn't otherwise know how to make or even think about. This can be a player who knows a beholder has an antimagic cone and has the character prepare accordingly.
The simplest solution is to prevent players from making a decision they shouldn't be told “no, your character doesn't do that”. A more subtle approach is to change the way monsters work so that they become completely new to even the most experienced players.
A middle ground is to allow players to roll Nature or Survival checks to deduce how a monster works based on study, lore, or examination.
1
Ask politely
The best rule of thumb for every conflict between DM and players is to have an out-of-game conversation that addresses the issue. If a player at your table uses an exploit that hurts the enjoyment of the game for other players, talk to them about ways they can change their character to fit in with the rest of the table.
Most of the time, power players want to show off a cool build they discovered or just want to feel unstoppable. There are other ways to allow this while maintaining the integrity of the game. This can be an intimidating question, but it's the best option for your long-term gameplay and table sanity.

Dungeons & Dragons
- Created by
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E. Gary Gygax, Dave Arneson
- The latest movie
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Dungeons & Dragons: Honor Among Thieves
- The first TV show
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Dungeons and Dragons
- Air date of the first episode
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September 17, 1983