so i used the plugin Devel: Generate to make some fake postings, but now i want to delete them all and there is 500 so is there a one click type way to just delete all threads in a forum?
In your database, using phpmyadmin, you could drop/delete all the rows inside of [Default prefix is mybb_ else respectively change it]"mybb_threads" table. That being said, just note that this will delete ALL and EVERY thread on your forum, including posts that are in the thread(they'll exist in the DB but not in public view[suggest you drop/delete those rows of posts too])
But I would wait for someone to confirm this as others know more than me on this matter.
PS: Make sure you have a full backup(Files + Database Backup)