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Is there a Group Policy that will stop users from downloading things such as Smiley Central, HotBar, etc.?
Not specifically. However, you can use a Software Restriction Policies (available on Windows XP and Windows 2003 clients) from blocking the download of any specific application's setup. You can create a hash rule such that the installation program cannot run. That ought to do it, though it will take a fair amount of elbow grease to hunt down all the installation programs you want to block.