i found that the best way of locking out the mech is to take off the circlip, take out the metal plate witht the tensioner screw, take off the black plastic washer, take out the spring, now put a magura washer that normally goes on to your v-brake mounts then finally put the circlip back on.
now put the mech on and tighten it up nice and tight so it doesn't move
i did mine this way and i still get 6 gears to work properly
i have a shimano 105 short cage mech by the way, they're about 30 pound and pretty solid