Simplest solution - kingdom that is losing should state the following:
"If you stop hitting/ops (except retals) now, we will surrender in min time. If you keep hitting while we have already conceded, we will consider it bad blood and will war till EOA in spite."
Assuming there is decent reason to believe them, then the winning kingdom has 1 optimal strategy for winning the war - stop attacking/opping. Anything else is clearly detrimental, and so this lack of hits is fighting the war to the best of their ability.

I wouldn't put it past them to punish the *loosing* kingdom, but then that's a real threat if 'tis past min time, so why can't it be made before then? And it isn't like they didn't just get smashed as punishment as well. But that situation would be one where punishing the winning kingdom is pure hypocrisy, since they were trying to win to the best of their ability by not attacking. (Collusion, of course, changes things... but afaik, collusion is only a problem if done in game, so just take it to irc or something. That one struck me as bad precedent too, but once made, can't unmake that rule.)