Marking threads as complete

[Purfleet]

Not sure there is an easy answer to this one

How can we encourge people to mark topic's as complete? There are a large number of posts (expeically in the excel forums) where it appears that answers have been given but users are not marking as complete?

I understand that some users will just take thier answer and never come back, but a lot say a solution works but never mark as complete.

Just makes the forum messy and incomplete.