You use a hacksaw to remove the old straplocks. Fucked up'dly enough, this was the least invasive method. Angle grinding/dremeling/precision drilling to destroy the stupid jammed part and get to the screw head would have been way more invasive and high risk to the guitar.
Fuck Dunlop Straplocks. Never use them. If that shit jams/rusts you are shit out of luck. Besides they are nowhere near as good as Schallers.
If you are one of the retards who uses this atrocious design. Take this opportunity to go and fuck yourself for putting atrocious shit on your guitars.