I suggested in another thread that successive downvotes on (1) one person's account (2) over a certain number of downvotes (3) within a set period of time should prompt the system to tell the user that they have to sacrifice personal karma until (x) days later in order to use up/downvotes.

Something like this is already in place, where a person has to sacrifice karma in order to comment on a post that itself is below a certain karma threshold.