If the motor is in the car you can just remove the front bevel cut balance shaft gear and leave the shaft in there stationary as a dead weight. If the motor is out of the car, you're best to remove it all together. The front bearing hole is just left open, but on the rear hole you'll need to remove the original 'white metal' bearing and block the oil feed hole. Yes you can use a welch plug to block it, just so long as its installed far enough to fully seal off the oil feed hole. The other way (my preferred) is to tap an 8mm thread in the hole and install a socket head grub screw. The way I do this is to use a 'starter' or 'full taper' tap and intensionally not cut the full thread depth all the way through. This way, when you thread in the grub screw, it will lock up and tighten on the thread which is not cut the full depth. Use a 10mm long grub screw and keep testing how far it threads in before cutting the thread any deeper. You also need to ensure that when the grub screw is fully home, that is doesn't protrude out the back to hold off the alloy rear main plate. Just for a little extra security, I tighted it up with some high temp bearing lock in the thread also.
Hopefully that makes sense and helps.
Cheers