How do you deal with code review cowardice?

The worst thing you can have in a pull request is approvers that will not bring anything to the code review and just wait that someone else approves it before approving it themselves. What's the point of their presence in the process if they won't add anything to the discussion? How would you deal with that? Do you politely ignore them? What if you're in a deadlock with the other approver and they still don't say anything?