I'll say it again 'delete the forums, then the category' but since you've already moved them they will appear as forums under the category to which you moved them in forum admin -> management so just delete them from there.
It's not the years honey, it's the mileage...
I do not provide any install services for phpBB, Mods or Styles.
Please do not pm me for support/scripting help - you won't get any reply. If you have a question then make a post in the appropriate forum.