Try a brand new 5mm allan wrench. i expect your problems came from using a worn wrench that had the sharp corners rounded a bit from use and it didn't get a good purchase the first time.

If that fails, and it probably will at this point, you could drill out the bolt using a drill bit just slightly smaller than the hole in the chain ring then chisel off the head which will be held by a very thin ring of metal. Another possibility is to cut a slot across the bolt head with a fine tooth hacksaw and try a large screwdriver to turn out the bolt.