Time for a new left arm. If it loosened up once and you didn't ride it much you might be able to use some loctite (on the taper interface) and tighten the hell out of the bolt to keep it in place for a little longer. But since this has happened multiple times the arm is likely completely ruined.

This is one of the few places where you absolutely need a torque wrench if you've never installed anything that requires ~40ft-lbs. It's a lot and you'll likely not get even close just guessing. Once you have the feel for it you may be able to go without the torque wrench.