You may find detailed instructions on how to uninstall Thea for Rhino in the following link.
Write down the path to the Thea Data folder before removing Thea for Rhino.
The Thea Data folder contains all of your Materials, Models and Custom LUTs. Make sure to make a backup of the folder before deleting it.
Even if you uninstall the Thea for Rhino plugin, the Thea Data folder will not be removed from your system. Here is how to find the path to the Thea Data folder: