Can I clear out my brush categories back to default?

Yes you can do this, although you can only achieve this by modifying a registry value. You can find the registry key at the following path:
HKEY_CURRENT_USER\SOFTWARE\Quixel\3DO2\ExternalBrushPath_$$$$$$

Deleting this value should do it for you! We apologize for the hassle and will make sure to include a simpler solution in upcoming releases.

Tags: DDO, Brush, Reset, Default, Custom, Remove, Known Issue