You can remove first gear and the OWB altogether. Then, pull out the clutch assembly and remove the shift point set screw, spring, gray plastic plug, and ball-bearing. File some round "half moon" spots at three locations around the metal hub and then screw three 4mm bolts into 2nd gear. The "half moons" in the hub should fit right up to the bolts for best results.
The clutch arm is out in this pic, but it really has no use now.
If you want to revert back to 2 speed, you'd have to buy a new clutch.
If you don't want to mod the clutch, b0gh0s makes a locking hub which replaces the clutch and does the same thing. His solution is totally reversible. Depends if you want to go the cheaper route.