Browsing our latest 50 questions, I only see two closed ones, and only one with a negative score, so there's certainly no problem. At least not yet.
That said, if you feel a question is soliciting debate then your options are:
Your first thought should always be to edit the post. If you feel the phrasing is a bit not constructive and you feel comfortable that you can transform the question into one that brings value to the site, go for it. Make sure to explain to the OP why you've change their question in the edit message.
Keep in mind though that editing becomes less of an option when there are answers, as your edit might invalidate them.
If you don't feel comfortable editing, or the only edits you can think of would completely change the intent of the question, then it might be preferable to just comment and explain to the OP exactly why you feel their question isn't suitable for the site.
If it's a new(ish) user, please don't just tell them their question doesn't fit, if you don't have the time to explain why that is, it would be preferable to not comment at all, let someone else take care of it. Pointing to our subjective guidelines might seem enough, but during early beta is extremely important that all of us go the extra step and explain to askers what exactly is wrong with their question.
Flag / Vote to Close
If all else fails, flagging or voting to close would be appropriate.
I may have laid out the options in order of preference, but what you'll do it's completely up to you, it's your time and effort after all. You may choose to comment and flag / vote to close, and that's perfectly fine. Even just flagging / voting to close would be helpful, you're signalling to moderators and high rep users that the question is somehow problematic (and that's all voting to close means).
Lastly downvoting is, again, completely up to you. Similarly to close voting, downvoting is only a signal to readers that there's something not quite right with the post. Keep in mind though that there's a reason we have two review systems, one for topicality and one for post quality. A question may deserve both a close vote and an upvote (for prior research), it's rare but it happens.
Close voting and downvoting only become somewhat synonymous at the very end of the quality spectrum, if the question is not crap and it's only problem is that it doesn't fit the Q&A philosophy and format of the site, then there's no reason to downvote it. Similarly, poorly researched but on topic questions should be downvoted, the fact that they are on topic doesn't say much about their overall quality.