Opera 9 has added a solid ad banner blocker, under the guise of a generic "content blocker". (This indirect name is presumably only there to keep a good corporate image.) To block an ad (either an image, frame, or Flash animation), right-click an empty spot on the page and choose Block content. Then, simply click the banners which you want to block. When finished, click Done in the pseudo-toolbar that appears at the top of the page.
By default, this tries to use some logic to block all ads from the source, but this may sometimes be overkill. To correct this, click the image again (to unblock it), then Shift-click it. This will add only the specific image to the filter list. This filter can then be broadened by clicking Edit.
A filter will typically contain asterisks ('*'). These mean "any number of any characters", much like in filenames for operating systems.
To edit and/or add to your list of filters, go to
Tools - Preferences... - Advanced - Content - Blocked Content.