Jeff, Our club has a scheduled training night every Thursday. I only teach on Thursdays. It sounds like you need to practice a polite but FIRM NO.

Crashed too many poorly built airplanes? If you're doing a thorough preflight you can probably pull out an elevator or aileron, oops I'm sorry you didn't get that glued in properly. Take it home and I'll see you next week!
Or I'm sorry I don't do test flights.

On your flying night; Jeff can you give me a hand? Your answer; "No, sorry not tonight, I teach on Xdays, and Xdays only. This is my flying night. When you start teaching you will understand my position." You only have to do it twice, and be consistent, and they stop asking.

HTH
Tom teaching for 6 summers straight now