To strip acrylic paint I use false nail remover bought from Supadrug because they usually have special offers on it so I can buy quite a lot for £5.
Pour it into an old glass, then bung the figures in that require stripping. Wait about 5 minutes and you'll see the paint start to bubble and peel off. Take the figure out and use either an old toothbrush or a nailbrush to vigorously brush the figure.
Some figures require more time in the solution so use an old jam jar, put the false nail remover in there, put your figures in then put the lid on and leave it overnight. Remove them the next day and give them a good scrub.
Note that this method only works on metal models because false nail remover will melt the detail off plastic figures in seconds!
Some people here will no doubt recommend brake fluid. I've not tried as it's difficult to dispose of. It's illegal, for good reason, to pour this down your drain.