[Group IM: How do I make sure I'm out of it?]

Just now, LittleMe Jewell said:

If you are a member of any group then you will get that group's chat by default any time anyone says something in the group chat.  Don't bother blocking individual people - it isn't their fault and it does not stop the group chat.  I don't use the regular SL viewer, so I don't know if you can turn group chat off.

Like I said, it's not an actual group, it's just like, a bunch of people that was IMed at once.

[Group IM: How do I make sure I'm out of it?]

So, someone I don't know selected me and a bunch of other people I don't know and started a giant group IM.  I blocked the person who started it and then closed the IM, but later when someone else said something new in the group IM, it popped up again.  How do I ensure that I don't get any more IMs from that giant group IM?