The easiest solution is to implement hard-coded buffs/debuffs into your role selection when you select a certain role/icon.
Think vengeance PvP passives.
Or generally PvP battlespirit passive.
If you select tank or healer icon, your damage output is cut to 1/3rd the output. If you select DD icon, your healing done is 1/2 the potency and your damage taken is doubled.
Rebalance dungeons around that.
Result: No more cheesing roles.