Please make sure you have a backup of any files you might edit.....
You can edit the Category.xml located at C:\ProgramData\QlikTech\ManagementService\QVPR. If you look below you will see I have two Category Names Default and Test. You would have to delete the line you want to get rid of.
<Category Name="Default" ID="728303e8-0a6c-4bb2-8057-4247ff85b6d4" ModifiedByUser="QVLAB\Administrator" ModifiedTime="2013-09-03T20:33:33.8055449-04:00" />
<Category Name="test" ID="522adccf-f644-4551-bcf0-5ee5767400de" ModifiedByUser="QVLAB\Administrator" ModifiedTime="2013-09-05T16:38:09.4918288-04:00" />