Are you sure it's the a/c shaft magnetization that's causing the problem? The a/c clutch is a magnetic clutch that would reasonably magnetize the shaft over time. How was this diagnosed? Perhaps the speed sensor is subpar.
If the problem is indeed excessive magnetization of the shaft, there are degaussing units available. If the a/c compressor is out of the car, take it to an automotive machine shop that does magnafluxing. They should have a degaussing unit in the shop. If the a/c is in the car, I can only recommend checking a TV repair house for a portable degausser used on TV picture tubes or an electronics store for degaussers used for tape heads. The ones for tape heads may be too small for what you need, though.
95 E320 Cabriolet, 131K