Simple green or Pin-sol
Just submerge the model in the fluid. Make sure to do so in a ventilated area as the stuff can be quite potent.
Also, make sure to keep checking frequently, as sometimes the model can begin to melt when submerged too long.
It can take over night or more though, so don't be too worried about keeping them in there.
When you pull em out, just use a toothbrush to scrape the paint off. A craft knife may be needed to get the small detailed parts (depending on how caked the models are).
Most of my Nurgle models were cleaned of their old paint jobs in this way. Good luck!
