The mods cannot define spam, unfortunately. By its very nature, spam is anything meaningless, irrelevant and/or unsolicited. Since those are, to an extent, subjective, then determining what is spam is itself a subjective endeavor. That is the current system in enforcement -- subjective moderation of spam -- and it really is the only real way to handle it. I think it's enough though, especially in your case. You were really spamming it up.

Here's a good way to determine if you're spamming: if you're only posting for the sake of posting, then you're already spamming. (Someone also already said that before, I think.)

You don't always have to post. Do you know how many times I've wanted to post on one of those superspam threads just to tell all those spammers what massive douchebags they are? Tons. But I don't. I hold my tongue and refuse the urge to post, because that in itself would be spam, in addition to being a hypocritical post. There are also times when I just wanted to "lol" at a post for being witty or likeable, but I can't spam the thread just to do that either. So what I do instead is quote the post and PM the poster. See what happened there? No spam, but I still got to express myself.

One-liners aren't necessarily spam. If someone created a thread to ask for something simple (yet still worth starting a thread for), then you might have to reply with a one-liner for that. In fact, rarely, even a couple of words might suffice. In that case, your post wouldn't be spam. But if someone has already posted the necessary reply before you and you just post it again because you didn't read the whole thread, well, guess what -- you just spammed. And that goes for multiliners as well. And of course, if your post doesn't help benefit (EDIT: 'benefit' has more practical connotations than 'help') anyone who reads it, regardless of how lengthy it is, then you're obviously spamming.

It's all subjective, really. I guess it's like American Idol -- oftentimes, one or two judges might not vote for you to go to Hollywood because of personal preferences, but they are not to blame if you're naturally tone-deaf. Just exercise your common sense.