If the players are dead set on finding a way around a challenge rather than facing it, put it on them to come up with a way to do it. If they want the establish something you haven't thought of, then it's a declaration.
I agree with Papa Gruff that such declarations shouldn't be to just let the players find open and unguarded windows or something similar. That sort of thing robs a scene of its drama and tension, and really, this game is all about the drama and tension of facing danger to do the right thing. It's not about the players figuring out how to get around challenges or outsmart the GM. Players and GM alike should be working together to make the game as varied and entertaining as possible.
Remember you're totally within your right to refuse a declaration or an assessment (or just set the difficulty very high) if the player is asking for something that isn't there or isn't appopriate to the scene.