When playing as PLD and Healer, if I mark targets and a DPS insists on attacking the wrong target time after time, I let that dps get punched in the face several times.... Low lvl dungeons are easy enough to 3 man.

I've gone as far as not healing a dps as whm/sch for entire runs to a boss and as pld i've let a mob go loose on a dps and then after the pack is dead I gently remember people to attack my marks.