Maybe it's because I'm using Lion, but in Keychain Access, once a certificate is selected in Viewer, it can be deleted via Edit (Delete) or merely by hitting the Delete key.
I tried that as well as various key combinations, e.g., Option & Command, etc. No joy.
As Jon noted, I can hide those rascals, but would prefer to resolve the issue versus live with a work around. And since this has no operational aspect, more akin to an academic/learning challenge, I may settle for a gentleman's C (at least for now).
