There isn’t an easy way to keep them from selecting the theme, but you could create a theme that checked the current user’s group and then did something like redirected people to a page that told them to change the theme back or to subscribe.
There isn’t an easy way to keep them from selecting the theme, but you could create a theme that checked the current user’s group and then did something like redirected people to a page that told them to change the theme back or to subscribe.