Chris,
They can be stubborn, even with the knocking block, and I have no experience of any other method. That “tool” designed to ensure the bars are not damaged when they are gripped so be very careful if you do try any other way of holding them. That said, they are only two rectangular blocks and a couple of bolts and the dimensions are in the manual so any half-decent workshop could make a set for you.
You might make the job a little easier by a liberal application of WD40 (other similar penetrants are available) and allowing it to soak in for a day or so. It will also help to remove any torsional load on the bar – i.e. remove the rear wheels (if you have not already done so) and support the rear hub at such a height that the torsion bar is totally free of any load.
B………