If you don't want to use your traps during a face pull you have to stop  pulling together your shoulders and if you do that then it's no more a  face pull. 

The face pull has many variations, but all of them have one thing in common; they are not rear delt isolation exercises, every face pull is a big compound movement that works many joints and muscles, they are the squat of your upper back. 

if you want to isolate your shoulder try rear delt swings or rear delt kickbacks.

Edit :
As for variations with less upper trap activation,try standing high pulley face pulls. In this variations the movement actually starts with the lower trapezium and the rhomboids.