how do I remove acrylic paint from my warhammer 40.000 models?

If you're in the US, then you can get hold of Simple Green cleaning fluid. Leave the models covered by the fluid for about 8 hours, then scrub gently with a toothbrush and you should find the paint simply sloughs away.

If you're in the UK or EU, it's a tougher call thanks to restrictions on cleaning fluid contents this side of the Pond. However, Phoenix Precision Paints sells SuperStrip - a cleaning fluid specifically designed for removing paint from plastic miniatures.

They don't have an online shop, but you can buy by credit card or cheque.

I'd add that, whilst acrylic paints are, indeed, water based, they are non-soluble once dry, so you can leave them in water as long as you like and all they'll get it wetter.

Tamiya acrylics are pretty tough. Water won't budge them. Try soaking it in a degreaser like Simple Green or Super Clean(Walmart).

If that doesn't work you may have to use Easy Off oven cleaner(non-foaming type) Use it outside well ventilated. You may have to scrub it with an old toothbrush.

Acrylic is water based. Soak in water and rub off paint. Be careful with your plastic figures.