Tell people things like, "Hey that's an interesting idea. Thanks for the feedback!" That is making no promises nor making any direct feedback on the quality (or lack of) for the suggestion. Of course if you really really think it's a good suggestion, do say so. The catch is, don't clearly gush over the good ones and say, "yea thanks for that (NOT!)" for the ones you don't like. People can subtly sense your mood there whether you think so or not.
Don't say "I'll look into it" (unless you really are going to), nor say things like "What a dumb idea!"
The key here is to acknowledge to the person posting that you saw their feedback, and that you appreciate their involvement in the community. That's what they really want to hear and see.
Think about how it is on this site. Suppose you post a question or an answer. It's very human to be curious how the community will respond to it. Get an upvote? That's great! Get NO votes? That's kind of a bummer because maybe nobody read it, maybe nobody cared. Get a down vote, and it's hard not to feel a bit rejected.
Also, be open minded. Good suggestions will come up in your forums. And with every suggestion, try to look beyond the specific idea and see if you can identify the root motivation to the suggestion. Maybe the aiming system in your shooter is actually not easy to use, and someone makes a suggestion about a change. It may be their idea really stinks, but their root issue with the aiming system is legitimate - and that's what you need to know. So you could turn that into a, "Hey thanks for your idea for a change to the aiming system" type response.
Lastly, this is why you need a person familiar with community management to handle such things. If you don't have such a person or aren't the person to do this, consider just not having an online forum. They can be more work than you think.