Well the trick is going to be not to provoke many AoO from that player, more monsters with reach that don't have to close, more ranged attackers, more spell casters, stuff like that. Once the first person gets eldritch blasted for even approaching him the others will be shy to do it, unless they swarm him remember only 1 reaction, so the first guy takes one for the team and the others just pounce.
If it becomes an issue and you want to back peddle on it show him this http://www.sageadvice.eu/2014/09/16/polearm-caster/ Mike Mearls saying it shouldn't work. Now of course that is a a rules as intended answer, the combo is perfectly ok by the rules as written currently.
Most of my players are optimizers, the way I deal with it is harder encounters and use higher hit point amounts for the enemies.
Good gaming.
When you have to start modifying your adventures to deal with one character, that is the definition of overpowered.