You can't completely disable the taskbar through group policy, only severely limit it. Sounds like you need to configure your kiosks to use a custom shell.
I've heard a few people just create an empty .exe file and point their gpo to it and it essentially is like running a pc without explorer.exe running at all. You can then configure your app to start on bootup. However I'm guessing users would still be able to launch explorer.exe through windows key + R.
The GPO location you're looking for if you're using Server 2003 (not sure about 2008 R2 but it should be similar) User Configuration\Administrative Templates\System\Custom User Interface.
I came across a post awhile ago about this at superuser. Explains quite a bit of the process you'd need to go through.