Okay, I have a plan:
1. We execute Knight Plesen because he isn't blocked.
2. At night, the Necromancer blocks the zero-level creature again. The Rebel will surely kill someone (probably a Mir).
3. We hang the Rebel.
4. At night, we block the zero-level creature. We remove the immunity to execution during the day. We block it again. We execute it permanently.
There is, of course, one flaw in the plan, but I don't even want to think about it.