Try this:
Add a
banner
to the skyscraper category (create a new one if you want)
Create a new billboard - making sure that the ad(s) in the billboard only select those from the skyscraper ad.
Edit the forumdisplay script. Untick the 'use master'. Assign your billboard created in the previous step to an availible $GAB_ads[x].
Put the $GAB_ads[x] in one of your templates
HTHs