My suggestions for revoking-the-comment feature:

• Don't actually prevent voting, just limit the extent to which the comment can be downvoted. Say, no lower than -4. Or at least -2. Otherwise, allow voting. Maybe the community decides that it's actually a good comment! (Another variation: allow unlimited downvoting of a comment, but make it have no impact on Karma. It's probably an even better alternative.)
• Don't indicate that a comment is revoked by striking through all its text. It makes it hard to read, which is annoying. It also makes it impossible to add clarifying comments about why the comment is revoked in the same comment, without the clarification also getting tainted.
• Allow deleting comments without children. This supports the use case of reverting something posted by mistake or recognized as stupid on further reflection. (You already know this one.)