I feel your frustation... Its hard to deal with people steping on our toes and all we can do is "growl" to ourselfs, wishing some curse on them...
Some members of the mafia can "kill" accounts, but its a bit risky, imvu might found about it.
Besides its easier for you to change the account for only to be permissive with buddies, including receiving mensages only from them.
Block her and the other guy/girl.
Tell your boyfriend to do the same, if they start to annoy you in the chat rooms just report them. It takes some reports to get her account deleted, and don't forget: Do NOT fight her back!!! Neither your guy, else you two can get into trouble.
Besides, even if some of the mafia members decide to "kill" her, she just might create another account and start to piss you off again. Its better to ignore her, she will quit sooner or later, if all else fails... I suggest getting another account.
However, if her homepage does have anything that is against ToS, I will help you by reporting her.