Your questin is rather vague, since you don't say what kind of paint, how long it's been on the model, and where it is, on parts, or a completed build..
If it's already cured, a product called "Simple Green" works quite well on enamel if the model's soaked in it overnight... As for acrylics, ammonia works, as does alcohol on some.. For laquers, I'd recommened sanding it off only, as any solvent strong enough to remove laquer will also remove your model...