I'd guess that the computational intensity of maintaining non-block "affinity blacklists" for each user doesn't have a corresponding payoff to make it worth doing. Similarly, I'm not sure people would want to "list ex-relationships", even privately within the system… For one thing, it would be a personal reminder… But sooner or later, the Zuckatroids would start using it to send you ads that would somehow relate back to that ex, and that would just be all bad.
Consider the occasional surfacing a "lesser of evils" – especially if you aren't willing to outright block her… Though, you may want to give that option, and why you're hesitant about it, some further thought.
Will Facebook ever be smart enough to not display ex's on the "People You Might Know" section?
deano.com 