I've been in that situation at work. I let the guy fly it...and it crashed. Probably the most violent crash I've seen. I chaulked it up as a lesson learned. My fault for being "dumb".

I won't just flat out say no. I let them know "it took me countless hours of flying time, crashing and hundreds if not at most $1000 to get where I'm at now". "Do you have the time and money to put into it at this moment?". 9 out of 10 times (without going further into it) they'll get the point. If they don't thats when you just have to say NO.

It's not being rude. They just don't understand.