I hate them. They just come on to your userpage and make random-ass derogatory comments.
Like so:

"OMG U NOOB!!111 LOL"
Ok, so I'm less experienced than your level 30 ass.

"Have sex with a lit stick of dynamite!"
I don't even own any dynamite.

"YIFF IN HELL FURFAG!!"
Do you see that little icon on my tool bar? That's google hacks. I'll steal your password and ask for a ban.

"shut up faggot"
You're banned now. And do you see my extremly well thought-out counter-insult down below? I suggest you do what I tell you and grow a pair. PLUS your username makes you very obvious, along with your choice of insult. Hello Joseph Kruise. GTFO Newgrounds and get rid of your free trial of flash. Stop playing R00nSkAPe.

" : : Fucking owned, bitch. x 10000"
Yeah, spamming someone's userpage is real cool. Go die in a hole.