First, quietly and respectfully ask other comedians if they agree with you. It’s not about accusing anyone. It’s about making sure that you’re justified in calling it a “theft.” If others feel reasonably certain that another comedian is using your material, then you can try talking to the accused thief. If you believe that the comedian might try to attack you back or so that you are the thief, make sure that you let others know that you are about to talk to him. Then other comedians will know that any accusations the accused thief makes came after you approach them, which casts doubt on the thief’s claims.
Again, when you talk to the accused thief, be respectful. It’s “innocent until proven guilty.” If they are truly guilty, then being polite won’t hurt you because you’ll have evidence to back up the claim.
If nothing works, you can bring your concerns to someone who’s above the accused thief. People who book shows tend to not want joke thieves on their shows. Not only might the joke thief steal their material, but having a joke thief on their show makes it harder to book comedians who are actually talented and work on their craft. If a book wants high-quality comedians on their show, they’ll also want comedians who don’t steal jokes. The purpose isn’t to “punish” the joke thief. It’s not about “getting back at them.” When done correctly, it should be the comedians inside the group saying that they are better off without the thief. It’s a factual statement that doesn’t require controversy (at least, from you).
The final topic in this video covers how originality can be an effective tool to preempt any attempts that a joke thief might use. When you develop original material, it becomes irrational for a joke thief to steal the material because A) It becomes painfully obvious that his material is related to yours and B) Original ideas tend to not fit well with generic comedians. If a generic comedian says 10 conventional jokes in a row and then follows it up with something original, it’s painfully obvious what happened. This makes originality the best tool to preempt theft.