Presume good faith on the part of your players. If they propose something, they think it's reasonable. You owe it to them to take that idea in good faith and think of how it might be possible.

If it's not possible due to things unknown by the players, it does not have to be accepted. They can find out the information later!

If there are requirements the players are unaware of, inform them of what would be required. The things that are required might not be possible if the task is actually impossible!

Outside of hidden information, treat the players thinking something is plausible, and you thinking it's not, as a matter of incomplete communication, and inform them of the things that make their idea implausible.