How to get out of group I didn't join on yahoo groups?

Where are you getting this? You didn't give those details and without them, rather hard to give a good answer. If you're getting posts in your Yahoo Mail, mark every last one as spam and create a Mail Filter to shove them into your trash bin until they start getting marked properly as spam and you never see them. If Yahoo is telling you in person they don't have this group but you can go to a domain like groups.yahoo.com/group/NAMEOFTHEBADGROUP and it's there - someone's yanking your chain at Yahoo! That means the group exists and look for every possible way to report the place as a bad group until Yahoo does something. If you have mail somewhere else, find out how to block the group's mail that way. They have to be sending from some sort of domain like @igottaspammygroup.com so find out how to get that blocked. If you're getting it from @yahoogroups.com then it IS coming directly from Yahoo and a Group that is really there.