SUMMARY Currently there's no mouse function assigned for 'must not have this tag' in Tag Filters. I suggest assigning 'ctrl + left mouse click on tag'(or something similar) for this function.
Another way this could work would be if the checkboxes had 3 visible states (neutral, "must have", and "must not have", as described in bug #418380), and each successive click on the checkbox would cycle it through those 3 states. With sufficient visual feedback, this could be quite discoverable, and would work nicely on touchscreens, as well as with clicks.
What's this feature "Must not have this tag" exactly ??? Gilles Caulier
(In reply to caulier.gilles from comment #2) In the Tags Filter, if the user right-clicks on a tag name, the choices for Tag Filter Mode are Ignore This Tag, Must Have This Tag, or Must Not Have This Tag.