This is pretty much my life. Every time a friend ask’s me to “brainstorm” I know its code for “agree to the ideas I’ve already created”. It angers me that I’ll actually sit there and try to come up with new scenarios and explain how the characters might handle the situation, but everything I throw out there is shot down for what ever they’ve already formulated and are really just wanting me say, “Gee, what a grand idea. That’s absolutely brilliant.”
I go through this a lot. It’s gotten to the point I no longer want any part of these brainstorming sessions or RPing for that matter. If you don’t want my opinion, don’t ask for it. Just pitch what you have instead of packaging it as a possible idea to explore. And if you don’t want to change anything about what you have, say so in advance so I’m not wasting my imagination thinking you want help.