How about a chance of spell failure when a spell is directed against a single player?
The idea being that stacked mages that have the potential to do 400+ damage per round against a single (player) target also have a, significant but not impossible, chance to fail cast (or the player can have the same chance roll for a save...whichever implementation works better)
Area spells would not carry this risk of failure.
This could help to reduce the one-hit-kill style hits some mages are able to employ but not reduce their effectiveness in player vs player combat totally.
On a side note, I'm also not a big fan of removing relocate / teleport to non grouped/guilded players. Add lag to these spells, yes, but remove their efficacy overall? I think it'd make it too difficult for a mage to hunt down targets and compete effectively in the pk realm.