it's called a vibration damper and if it doesn't slide right off you'd probably need to use a gear puller of some sort to pull it off of the shaft.
the only way to prevent the crank from turning is to lock the flywheel by placing a metal bar between the teeth of the flywheel and the engine block in such a way as to prevent the flywheel (and in turn, the crankshaft) from turning. pretty easy to do with the engine out of the car, but i've never tried it with the engine in.
you'd have to see if there's an inspection hole in the bell housing that would give you access to the flywheel ring-gear.
you'll also want to have someone around to help you with this.
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'88 R107.048 441/409 leder; Euro lights
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'70 R113.044 050/526; factory alloys; Euro lights