Can't really keep track of that, SP is just a unit getting moved around. what you can do is maybe add a "temp ban list" of posts to each one of the parties accounts with those urls so if one of the parties from say 100 accounts that got delegation from from 1 account voted while having delegated steempower the with the undelegation all accounts affected will not be allowed to vote on said post until payout. Problem is that this could be a huge loop to process so his method of correctly calculating voting averages still seems more realistic, and say the undelegated account has not used the delegation for a bit then you could get your delegation back at full power much sooner.