Louisa, the fact that this is an open site means just about anyone can come in here and post anything. All the helpful and friendly people you've met here can't do a thing about the bad stuff, other than ignore it.

I suspect the insults and criticisms without any substance come from people who are somewhat desparate that others notice they exist. The anti-whatever nature of their posts may make them feel like an individual, as if disagreement automatically makes them more important.

Just look at the criticism. If there's no content, and it's just something like "that's a dumb idea," you may be dealing with someone just looking for attention - maybe even someone's lonely child who got on the family computer and found a new way to write graffiti. They aren't talking "to" people; they're talking "at" people. Is there anything to indicate it's not a pissed-off-at-the-world kid?

As to whether or not you hang around, I guess it comes down to which things have more importance for you: the helpful advice, friends and potential friends, or the graffiti-writers. Hopefully, the "good guys" will win.