I searched all over the web for how to uninstall Keep Afloat, but came up with nothing. It is not a traditional application or service; it is a SIMBL plugin.
Uninstalling it is easy. Open up Finder and navigate to ~/Library/Application Support/SIMBL. Here you will find the Keep Afloat plugin. Just delete this file and Keep Afloat is gone.
If you don't using SIMBL, then you can uninstall that too. Go to the SIMBL homepage, download SIMBL, and run the uninstaller in the ZIP file.