First off, I am definitely in the large camp of players insisting that this 'Haunting' effect will only apply to Epic Hard/Elite, and nowhere else! Leave the rest of the game alone...
I do have a suggestion, though:
Why not have the Haunting DC-penalty be based directly off the caster's current DC?
Let's say, CurrentDC / 10 + 1, with a timer ticking off in 15-second cycles.
Each cycle of the timer could restore, say, 2 DC to the floating DC number.
A floating DC of 40 to 49 would result in a penalty of 5.
A floating DC of 30 to 39 would result in a penalty of 4.
Have the absolute minimum be DC 30 where you see any penalty at all, and have the lowest possible floating DC be 26 (DC 30 - 4 penalty). If anyone manages to kill a single monster on epic anything with a DC of 30, I say congratulate them and move on.
For example, if a caster has a Necromancy DC of 45, and successfully hits with an insta-kill spell, their Necro DC drops by 5 (45 / 10 + 1), to DC 40, for 15 seconds.
Now, if in that 15 seconds, the caster successfully hits with another insta-kill spell @ DC 40, an additional penalty of 5 is applied (40 / 5 + 1), dropping the caster's DC to 35, and the timer resets to 15 seconds again.
Now the caster lets 40 seconds elapse without landing another insta-kill spell, allowing the 15-second timer to cycle twice, restoring 4 DC to the floating Necromancy DC, leaving it at DC 39.
The caster successfully lands another insta-kill, and the DC drops again, leaving the DC at 35 again.
Etc etc.
The 15-second timer would obviously continue to cycle until your original ceiling DC is restored, given enough time.
A weaker caster who allows at least 30 seconds to a minute elapse between insta-kills would see little to no penalty at all.
Some stronger casters would be able to run with a floating DC a little lower than maximum, and allow their DC to restore to full potential during the lulls.
But it would at least slow down the spamming -- successful spamming, anyway.
Just a thought.