The number of times the banner appears can be controlled from the "banner properties" section in OpenX.
The deactivated banner is showing up because of the OpenX cache. (It is set by default to 1200 secs i.e 20 mins)
If you have admin access to OpenX, you can reset this cache period to a smaller value.
If you want to delete the existing cache immediately, then you should go to the server where OpenX is installed and delete the contents of openx/var/cache inside the openx folder (i.e where openx is installed)