There are no options in the control panel to delete them.
Just turn them off is the only stock option.
Untested but one option would be to edit the database.
The table is portal boxes.
But I would be extremely cautious.
All the island types are in that table.
In the future you may want to look at a banner rotator like
http://www.spyka.net/scripts/javascript/simple-banner-rotatorIt allows me to add multiple ad's per island and add them anywhere you wish.
And all the settings can be edited in the ubb control panel.
One site I know that uses it is at prosanddrags.com
the header top right rotates specific ads and the top right island displays them all.
But it could be dispersed over several islands and in the footer and decide if you want them all unique ads per location or per page.
I added it for them but they manage the way it displays.
They don't take advantage of all the location,display features.
But it works well with UBB.
And it is all contained in the UBB control panel.
Best part it is free.