In previous versions of Xoops, in order to display banners, you "had" to have a banner image to go with it.... that didn't go too well for those who do the banner exchange thing and had a javascript, or some html supplied by the exchange.... so, in the current verison, the ability to use HTML instead of a banner is now supported..... which brings me to the answer to your original question.... instead of adding the banner, simply use the a href = "your site" w/o the target element, and you should be good to go.
DISCLAIMER: I have not had the chance to try this, so I cannot/will not guarantee it will work. It is based on what I suspect to be true.