I believe you can delete the subforums or forums and the threads will be gone.
no dude, i created some forum and category (100-120)
so i want to delete them in same time rather clicking again and again
go into phpmyadmin and delete it there.
Log in to PHPMyAdmin (Done automatically if done through cPanel)>Click on your Database>Look for mybb_forums and click the hyperlink>Click on Select All>Delete or drop tables (Found in the dropdown).
Note: Your prefix could be different which means it might not be mybb_, it can be something else.