If someone commandeers a meeting (class, training session, etc.) that you are conducting, it is perfectly acceptable for you to do any of the following:
1) Leave the room with a dramatic door slam.
2) Make childish sing-song noises while the offender is speaking ("La! La! La!")
3) Do an exaggerated impersonation of the offender so that the person can see how he or she looks to everyone else (e.g., "Hey everybody! Look at me! I'm Pat, and I'm taking over this meeting!") You might also talk in a baby voice and wave your arms wildly just for effect.
4) Squirt the person with a water gun. (It works for cats.)
5) Be passive aggressive. Sigh and roll your eyes a lot. Revenge is a dish that is best served cold.
6) Go crazy. Start cursing and spitting and throw a real tantrum. You'll be the talk of the library for a while, but it might scare people into not interrupting you again. [Warning: people might get a kick out of seeing you "go off," so this might backfire and cause more people to interrupt just to see your reaction. Use this strategy sparingly.]