I can't believe this thread didn't get any play...
http://www.gofuckyourself.com/showthread.php?t=744871
Must have been too technical for most WMs...
I don't really know what you can do with a toolbar, but if you can trigger additional calls to the site in question, then there is NO way to stop this. Even so, if their toolbar auto-updates with a database, the lost traffic will still be significant.