Screwdriver between the clutch drive cog and gear it's driven with, in gear and broom stick through the rear wheel spoke locks it. You can undo the center nut with a brass punch(or if you are gash a screw driver-last resort really) and hammer.
Friction plates can get glaised and just need a bit of a scuff up. Tis worth replacing the springs(length should at least be checked to see if they're within tolerances) or if you're cheap use some washers to give extra tension.
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