I used to use
Fairy Power Spray. Soak the plastic, leave it a couple hours (or even overnight), Krylon turns rubbery and can be scrubbed off with a washing up brush.
A couple of notes:
- I haven't stripped Krylon off in quite a few years, so I don't know if the Fairy Power Spray is the same formula now as it was in years past. But still it's worth a punt for a few quid from Wilko.
- Generally, it doesn't attack the plastic, but you'll never get the colour back to deep-black - it'll be more charcoal/cloudy black if that makes sense? I wonder if car dashboard restorer would help.
- It can take several soak-scrub-rinse cycles to strip it all off, so it's not a quick process.