Several companies made a "spiderless" crank, in which the big ring was actually attatched to the crankarm and the small ring then bolted to the big ring. Stronglight made one, and I've seen several Japanese ones. Short of destroying the crank, I don't think there's a way to get the big ring off.

These cranks were usually standard equipment on lower road end bikes, mid eighties. Despair not, you can probably find an old Sugino or Sakae crank with an actual spider cheap.