That chainring is riveted to the spider (aluminum center) and is held on by a lockring on the backside of the crankarm. The middle and inner ring are bolted to the spider. If you wore out the outer ring ,you had to replace the spider. Unfortunately they are no longer available from Shimano. This however does not help help in your situation. Since I assume you want to use the middle ring, your best option is to find another crankset. I have seen people grind off the teeth on the outer ring and use it as a bashguard. I would also file down the shifting ramps so they don't accidentally catch the chain. You could always cut the outer chainring off. Both options are permanent and won't let you go back to use it as a triple later. Good luck.