HP delegations with decline_voting_rights
Declining voting rights was supposed to prevent stake of the subject account from being used for voting. However there is a hole in that protection, because user can still delegate HP to other account [under their control] and vote with it on posts. There are currently only 10 accounts that used decline and two of them have single outgoing (very small) delegations (at least one of those is our test account), so we can just clear existing delegations when HF28 triggers.
There are two ways of dealing with new declines in context of delegations. First is to copy-paste solution for RC delegations, that is, when someone executes decline and it triggers after 30 days, delegations will be automatically cleared, and in case there is too many of them to clear in single step, special object is created to continue clearing in following blocks. While RC delegations had no alternative to that approach (because clearing event could be triggered by different account than the one delegating), in case of decline we can use different way, namely we can prevent decline from being used as long as account has outgoing HP delegations. User will have to manually remove all delegations before executing decline, pending decline will also prevent new delegations from being created (unlike votes that only stop working when decline triggered after 30 days). That approach has two advantages - less work and it makes it clear to the user that he won't be able to delegate stake way before decline becomes final.