Hmm. I would create another OU & block GP inheritance on that OU.
This is what I would do as well. More specifically move the computer to the OU, as opposed to the user, as WSUS policies are applied to the computer settings.

Depending on what other policy settings you have, you may want to link certain GPOs specifically to the new OU. I would double check to see what settings are applied to that computer with Group Policy Results once you have it set up, just to make sure all of the right policies are applied.