Desolder the grounding plug from the motherboard.

Make sure your soldering iron is tinned and carefully touch the tip of your iron to the grounding lug. Allow the the solder to liquify. While liquified, lift the motherboard off the grounding lug.

WARNING: Do not allow the solder to spill over and bridge any connection on the motherboard.

New to soldering? Look at iFixit's soldering guide here.