pootle - there's no single set of rules or etiquette, but the rules at the beginning of this thread are helpful.
If someone likes you and you don't like them back, then ignore. If you respond with a 'thanks but no thanks' message, there is a chance they'll get pissed off or bitter, and you'll end up having to block them.
If someone messages you and you're not interested at all, then don't reply, for the same reasons.
It's harsh, but easier than having to justify why you don't fancy them.
From a mans point of view, if I message a bunch of women I find attractive, most of them don't reply. Many of the, don't read the messages (but I realised many of them were inactive profiles)
Then when someone does respond I think 'yay' and I read it, only to be told they're not interested. And sometimes listing my flaws (I have kids, I'm not taller than 6 feet tall, I'm divorced etc)
And that annoys me. I'd rather they just didn't reply.