When I leave my entertainment contract in 2 weeks how can I ensure they do not use my characters for other people who take my place next year?

I plan not to renew my contract with a local outdoor theater. When I do I plan to join a storytellers guild so the characters I have created will be used for gigs elsewhere. My fear is that the theater company will hire another participant to play any or both of the roles I have created. Part of these roles involves storytelling. I wish to branch out as a professional storyteller and these characters I have created are 2 of three I have created for storytelling. I feel like maybe I can get something filed with an attorney that will prevent them from doing that. I wonder if it is even necessary but I am worried that if someone is performing my roles at this outdoor theater that it may effect any work I may receive as a result of guild membership.