There is an additional, undocumented parameter for this command; if you set all=true then it will clear all tags. I just tested it like so:
Note that tagName is still a required parameter and it does have to be the name of a valid tag, but it will clear all tags from the given system. I will add a defect to document this parameter (and I suppose fixing this command so that you don't have to specify a tagName if you're clearing all tags would be nice, too).
Thanks Travis! This will save me quite a bit of time...