The way I've learned to deal with this is that I listen enthusiastically, and then I ask the person with the idea to come back with sketches or something more concrete. That has, so far, eliminated 100% of the ideas outright. When someone really serious comes to me with a concrete idea, I'll know it, and then we'll talk for real. Asking the person with the idea to do a little work to make it real is a great way to determine how serious they really are.