User added to a specific group should be blocked
Not sure which tokens you have available to select from, but the token site:current-user refers to the user performing the add to group, i.e. some "admin/'users with permissions to add to group". So your Rules Condition seems to be wrong. Due to which your Rules Action is not performed either of course.

And if the list of available tokens doesn't seem to have any relevant token, it may be because you're using the wrong Rules Event.