You can either:
1. "Modify" the shield with a hammer or other favorite beating tool.
or
2. Adjust your rear steering stop on that side. There is a bolt with a metal/plastic cap on the lower rear of the knuckle that you can adjust a bit to prevent the tire from getting too close to the shield. Just loosen the jamb nut, unscrew it a bit, and tighten the jamb nut. Test and repeat if necessary.
Personally I like option #1.