There's no easy / obvious way of deleting all the tags known to digiKam at once ! I can only delete them one-by-one... Reproducible: Always
A tags manager is planed. See bug #263299... Gilles Caulier
There is no, additional option for deleting all tags, but with Tags Manager, multiple selection is enabled. Ctrl+A, deselect root tag, right-click and delete selected.