Don't nerf the DPS, nerf the tanks. Specifically their aoe taunts. People can only speed run because tanks are able to reliably hold aggro on 6-12 mobs at a time while the enemies are being pelted by non-stop aoes. You can nerf their damage all you like, but as long as mass pulls + aoe have better potency than straight up single target DPS, it'll never end. Honestly, I think that even allowing AoEs outside of crowd control like Sleepga or tri-bind was a mistake in the first place.
These suggestions are aimed at allowing tanks to keep their ability to control small groups of monsters (with WAR able to do so slightly better, since that was supposed to be their gimmick) with bigger pulls becoming increasingly difficult to manage.
Flash - Off global cooldown, new cooldown 20 seconds (Allows them one free pass to get aggro, but then they have to swap targets to maintain enmity, good luck with your aoe spam)
*or*
Flash - Single Target spell, 10 second cooldown. Blinds enemy and generates moderate enmity on a single target, grants bonus enmity on targets unaffected by Rage of Halone's debuff. (Considering how easy the Devs seem to want to make tanking, this might be a bit of a stretch but it should be possible to handle a group of 3-4 with this. Worst comes to worst, PLD's learn how to use bodyguard)
*or*
Flash - Same as it is now, but now applies a stacking buff to enemies creating heavily dimishing enmity returns for every flash. Wears off after 10-15 seconds allowing full enmity gain again....or just make it so it can just flat out only generate enmity every 10-15 seconds. (PLD can keep their ezmode enmity generation, but mechanically sound PLDs will probably find they have trouble holding aggro on more than 4-5 mobs against BLM's spamming flare. For less skilled PLD it will be impossible)
Overpower - Can only hit up to 3 monsters at a time. (Any WAR who can hold aggro on 7-8 mobs with somewhat unreliable aoe enmity generation deserves the speed run)
*or*
Overpower - Diminishing enmity returns for every subsequent use.