Aspen whitewater shower won't turn off?

In house built by Bryants 1995. I can take the white plastic cover cap off and n then see the fitting. I think the thread may have gone in the cap or on the actual brass fitting however I am no expert ! . I have placed some tape over fitting to try and get a grip but only works for a couple of days. Any suggestions how to mend or where best to find replacement parts . Thanks

Glen
January 2015