At the moment, you can only do this service-wide, under services->review services->your tag service->perform a service-wide operation. I have a job in my to-do list to make the window that button launches more user-friendly and also allow you to perform its actions for the current thumbnail selection, which should do what you want.
I am not sure when I will get to this, but I will make a note to bump it up.
Although now I think about it, I can add a simpler 'remove/petition all tags' button to the manage tags dialog really easily this week. I will do that.