If permanent ink there's only one way to get rid of it and that is to scuff off the surface of the leather it it on then smooth out the scuffing. If it has penetrated below the surface it will never come out. I had a natural tan colored leather sling from Springfield Armory that had a bunch of stains on it that I covered up by getting some brown leather dye and dying the whole sling. To be honest, it looks much better a deep rich dark brown than natural colored anyway. After I dyed it and rubbed some neets foot oil into it it looks like a vintage sling and compliments my old rifle nicely.