My site has 4-6 older forums with posts that I would like to delete (have no wish to keep them as archived for search or similar). How do I do this properly and quickly?

Just go to yourforums/admin/viewboard.php?forum=xxx where xxx is your forum number and click delete forum at the foot of the page.

Or if you do not know the forum number go to forum settings within the CP, click on details of the forum, and click delete forum at the foot of the page.

Jesus, so thats where they hide that button, must have been blind the first time around as I did of course look there first... was just looking for some other icon I guess thanks

No worries - easy to miss - but then again, we do not want people accidentally deleting forums