If you're careful a crescent wrench on the tab in the middle will do to loosen them, but they may break if the plastic is brittle - adjust the wrench tight, and push it down as you turn, hope it breaks loose without much force. Those forks weren't worth much before they got rusty, I'd be inclined to give the stanchions a scrub with oily wire wool, the legs a shot of oil, and chuck them back together. (I had some similar RSTs that weren't rusty, they did respond well to a clean and grease, but they were too flexy so I replaced them with a rigid fork).