Be careful not to run java scripts in your orkut login. A big list of unwanted communities get added up to your list and you cannot even unjoin them.

if something like that has already happend then try this..

change your orkut password, relogin and unjoin them. Try to find the main community whose name was there in the java script. If you unjoin that, all the rest gets done , otherwise unjoin one by one.