There are two official ways of 'clamping' the engine and loads of unofficial ones.

a) buy a piston locking tool (these screw into the glow plug recess). a lot of manufacturers don't like this as they say it damages the piston.

b) buy a crank lock. You'll have to take the backplate off the engine and insert a tool that clamps things in place. This is O.S. preferred method.

c) other ways I would not reccomend involve shoving things in the exhaust port, or removing the carb and shoving things down the hole.

Sounds like you will solve your problem without resorting to any of these.