Acrylic or enamel? If it has cured, you may never be able to get it all out.
If enamel: immediately scrub the affected area with paint thinner or lacquer thinner. Do this outside while wearing rubber gloves. If that doesn't remove all of it, try soaking in the same solvent for 24 hours in a closed container (leave it outside). Finally, clean with one of the citrus cleaners (such as Gojo.) Then allow the pants to air dry completely before washing with strong detergent, or a soap like Murphy's Oil Soap.
For acrylic, I recommend trying "Magic," invented by someone on these forums: 1 part Windex, 1 part Simple Green, 2 parts water. You can fortify this with ammonia, but do that outside. Then launder with Murphy's Oil Soap.