What Leadfingers said. If the buckle will go through a loop, you merely scootch it over one. If the buckle is bigger than that, you have to rethread it. Besides solving the scratch problem, this eliminates the extra thickness pushing the guitar away from you.

I cringe seeing the suggestions to shorten the strap. Rather like telling a motorist to move the seat back so he can get a good view out of the mirrors--you adjust a car seat to where you can operate the pedals and steering wheel, then find a way to adjust the mirrors. Likewise, you adjust the guitar strap to where you have the correct reach and hand position, then find a way to avoid buckle scratching. You sure don't have to screw up your holding of the instrument to solve the buckle problem.