You could try some paint stripper or “Goof Off”. Use a little bit directly on the paint, let it sit for a few minutes then brush it with a stiff nylon brush. You may have to repeat that part a couple times, then when it looks the way you want it, rinse it off. Make sure that you use rubber gloves and do it in a well ventillated area. If you get any of the paint stripper on you, rinse it off IMMEDIATELY!
After you get the jeans looking the way you want them, rinse them well then run them through the washer a couple times to get any residual paint stripper off. It’s nasty stuff that you don’t want in contact with your skin, but it works well getting paint off.