I think "Dredmor" would work, as the taxa checker seems to check both the name and the actual taxa value.
^^ This. Also, probably include 'Brax', too. There is, somewhere, (hard) code for not affecting named monsters, because <effect type="removemonster"> doesn't work on named monsters. But it has yet to have an XML hook added to it. (pssst...Whitman...)
Then how would I go about making a spell that didn't effect Dredmore, Brax, and Count Diggula (or whatever his name is)?
Make a taxa-based spell which will first check if the target's taxa/name is either of the above and place a dummy buff on you, and then works normally but only if the buff isn't on you (removing the buff afterwards if it is there).