I just had a fork sandblasted and powdercoated, and now my speedo sensor won’t work, but only whilst installed.
Can this process cause the fork to become magnetised? If so, is there a way to de-magnetise it?
that’s possible I suppose, in the powdercoat process, the part is given a charge to attract the charged powder particles.
the best way to ruin a magnet is to hit it with hammer a bunch of times but might not be prudent for your fork 
turn it off and back on at the wall.
stroke it with a graphite rod or something? that’s what we did at school.
or, just don’t run a bike computer. you could gradually acquire a collection of small metallic road debris as some sort of hipster bling.
run the speed sensor on the back wheel