The ultimate simplest way is that if have full time sponsors of each forum -no banner rotations - this can be done with a simple "if" type statement which other members can help you with....
Rotation and Fractional Banners....
OK, this has to be slightly technical......but you can probably get help here or elsewhere with the final implementation.
Just for a description, let's say you were a remodeling site and had three forum sections:
Roofing
Siding
Kitchens
You want to be able to run banners in each one tailored to that particular section. A common use (and what I do) is have one ad at the bottom and one at the top of each of those forums. Then, in addition, you can sell "fractions", so you could have four sponsors in each forum, each with 25% of all the banners run.
There are many ways to program this in.....I am not familiar with Robbo's XF add-on, but you can check if it does this.
But it can be done with most any external ad banner rotation script also. I use an ancient one - from 1998 or so - and the things still works! There are many of them out there, including paid ones like adpeeps:
http://www.adpeeps.com/
OK, if you are still with me, there are two parts here.
1. The external ad management program
2. The insertion of those ads into the proper forums
What I use is a piece of code which says something like this:
"Make up a URL that pulls in this particular banner if we are in this particular forum"
It would be confusing to you for me to detail it further - but it can be done!
On my site I go a little further and say "if there is a banner, load it up, if not put a google adsense banner in there"