Tom, I had the same question. Maybe my solution will help someone else.
(By the way, perhaps it's not the same one to which you referred, but I remember an Aquarium screensaver from my Commodore Amiga days. It was a nice, graphically intense program, especially for that period. After Commodore went belly up, the author updated and released it for the XP platform.)
Anyway, a Google search sent me to the following page, windows.microsoft.com/en-US/windows7/Delete-a-screen-saver, but their suggestion was to go to the Control Panel/Programs and Features List; however, it did not include my new screensaver. From my experience with XP, I remembered that screensavers were stored in the C:\Windows folder\, so I checked there. Lo, and behold, I found a few screensavers, although their filenames were not the same as in the Screen Saver Settings list. I right-clicked on one of them. The highlighted menu item was "Test" so I clicked it and a screensaver preview appeared. The next one I clicked brought the one I wanted to get rid of so I moved it out of the C:\Windows\ folder. Voilą! It disappeared from the Screen Saver Settings list!