One particular incident happened to my friend way back, maybe around 2007-2009? Can't exactly recall. Anyways, his password got bruted (he believed it was a keylogger) one day and lost all access to his account. One could safely assume that a sane person who just committed virtual B&E on some random dude's pixelated doll account would proceed to clean house, but this was not someone who was sane. The guy who stole my buddy's account simply rearranged the items on the avatar and... Did absolutely nothing else. I'm not privy to every detail, so I'm not sure why it took the mods so long to unlock the account and ban the offending party. What I am sure of is that when my friend tracked down the account-taker, he received a series of bizarre PMs that talked about, and I loosely quote, "Burning (friend's name) in his light". The kind of odd, nigh-incoherent s**t a serial killer would type, if he was an otaku-fied teenybopper.
Not sure what happened to that guy. My friend got his account back, at least.
As for my experiences? Was briefly annoyed on a couple of occasions by someone who was not even trying to cover up how young he was. He did it every time I had a female avatar. First time he got agitated when I told him I was a dude, said that my profile said I was a girl. That bit of information didn't stop him, he kept PMing me and sending me pics of random women off FB he fantasized about as dominatrices and their feet. I told him over and over that s**t was making me uncomfortable, he'd apologize but the cycle would almost immediately begin anew.
I just stopped replying to his PMs, which I think got the point across. Only reason I put up with it for so long was because he seemed like a guy who was just looking for a friend to talk to. I was an idiot for trying, despite the red flags.