Once upon a time you could pay to have Nabbl's ads removed from your forum, so you can find historic posts on Nabble Support that refer to this, but there is no facility for that these days.
Now you need to create your own site, place your ads there and then embed your forum within that site. Nabble will continue to display ads within the forum itself.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.