near the shaft there are 8 or so holes full of ball bearings

remove all of those and either put a screw down there (not to far or your mess up the plate) until you can only move the shaft side to side a little bit

the best option is to get 2 grub screws per motor and use them as they won't pretude out the gearbox

i think the size you need is 2.5mm grub screws not sure

alex