I have been through this. Breaking fluid gives good results without being too messy. Don't bring it in contact with water or it will produce some nasty goop that is hard to get rid of. Only scrub the figures with soap+water when your are done.
Some of those old acrylic colors are really nasty. If breaking fluid fails, Dettol might do the trick. But it is messy and stinks. It is my weapon of last resort.
Simple green is often suggested but i don't have experience with it.
If the figures are painted with enamel colors your are out of luck. Better get used to the paint job
I was using Dettol a lot before discovering breaking fluid.
Dettol has a lot of drawbacks - it stinks, only use in well ventilated areas - it's not good for your hands, always wear rubber gloves - it produces a slimy goop that needs to be carefully removed by toothbrush - the goop will stick to the toothbrush and you will ruin some brushes until the figure is finally clean
Breaking fluid on the other hand - does not stink - is oil based and i never had problems using it without gloves (you might want to download the MSDS before using it though) - it creeps between the paint and the figure surface and the paint will simply peel off. No goop, and scrubbing with a toothbrush will not ruin the brush.