Many of these home remedies will work, but why not do it right and not take a chance of ruining someone's paint job, let alone the appearance of their truck ??
If it's a little 1/2" stripe, just tape it off a smidgen above and below the painted stripe. Sand it down with 600 or 1200 sandpaper. Go slowly so you don't go down to the primer. Remember, ya just wanna remove the stripe. Once the stripe is gone, hit it with clear and it will come right back. Ya might hafta feather it in, but according to what you're replacing it with, maybe you could incorporate something to take any unsightly yutzes away.