Just to clarify, the SEP12.1 inbuilt third party AV removal tool and SEPprep are different tools.
The inbuilt tool is supported by Symantec and enabled by ticking the "uninstall" checkbox in the SEP Client Install Settings Package. Whereas the SEPprep tool is found in the Part2 download from fileconnect under the Tools\NoSupport\SEPprep folder (take a look at Brian's link for more info, and "Thumbs Up" to Brian).
If this also fails to work, then you could take a look at the SEP Integration Component (SEPIC). The below article on the SEPIC provide a bit more info on it:
http://www.symantec.com/docs/HOWTO73212
One of the great things about the SEPIC, is that you can extend its ability to remove software by updating an XML file with atributes of the software you want removing.
http://www.symantec.com/docs/HOWTO10184