You can leave a community by going to the Community Profile page of the community you want to leave and clicking on the "Leave" link in the upper-left corner of the page. You will be asked to confirm that you want to be removed from the community. You will also be given the option to remove the community from your Friends list
The Community Profile page is located at http://communityname
.livejournal.com/profile, where "communityname
" is the username of the community.
If the community you would like to leave has been deleted or suspended and you can't view the Community Profile page, you can leave it by going directly to http://www.livejournal.com/community/leave.bml?comm=communityname
" with the username of the community).