I usually just say politely that it looks like we aren't a match, and I wish them luck. No snark, no unpleasantness... or at least I try to avoid it. The one time I gave in to snark in the last year or two was a bi guy who obviously didn't read my profile and was just spamming everyone within 20 miles, although he did say that he hoped I wasn't offended that a bi guy contacted me. I told him the truth, that I wasn't offended at all by being offered a BJ by a dude, but that I was offended that he obviously didn't read my profile and was just spamming, and really didn't give a shit about the people he was messaging. Honestly, I kind of felt bad afterwards about giving in to snarkiness because I'm better than that. I much prefer to be polite and pleasant, but we all have weaknesses.
I've had several folks ask the "why" question after I decline (always SFs for some reason). In that case I just say I have my reasons, and state my preference that we simply part on friendly terms. Usually that takes care of it, although I've occasionally gotten the tantrum in response. Which just reaffirms my judgement that we weren't a match in the first place. At that point, I just go silent or block. I don't need any more negativity in my life.