So it's important to say "no", but there are a few clarifications required on what I really mean, there. I'm not saying that every suggestion is wrong. In fact, developers are usually very bright people, and sometimes they really do nail it. Many developers make perfect suggestions and do excellent implementations. And even the worst solutions can have good parts, despite not being excellent as a whole.
So many times, instead of actually saying "no", what you'll be saying is something more like, "Wow, there's a part of this idea that is really good, but the rest of it is not so great.
We should take the best parts of this idea and build them up into something awesome by doing more work on them." You do have say no to the parts of an idea that are bad, though. Just because one part of the idea is good doesn't mean that the whole idea is good. Take what's intelligent about the idea, refine it, and build good ideas around it until the solution you've designed...