How about implementing a mechanism to prevent vouch challengers in PoH v2?
A way to do this would be to have a wrong vouches counter for every human which gets incremented for the voucher every time he vouches for a submission that was successfully challenged. Once this counter reaches a threshold (say 3), it’s not possible for that human to vouch any more. The counter could be reset when the person reapplies (or not). This solution has the advantage that it would be very convenient to implement, it is written in the code so there would be no confusion around it and would discourage vouch challengers (and people vouching without checking in general).