HELP!!!! strange IMs on yahoo messenger?!?

Suddenly a bunch of IMs pop up saying words like "hell" and constantly buzzing me. I tried putting it in the ignore list but it doesn't help. For instance, the first time it popped up, it's from "vsb_crazy_chick_1" then next one is "vsb_crazy_chick_2" and so on. It happened twice since yesterday already, and every time it happened, it lasted for about 50 IMS. Anyone knows what's going on here? Is it some kind of virus?? Any help would be appreciated.