Soak them in something like wd-40 and let them sit for awhile. Then get a paper towl around the spoke and grip tightly with a pair of vice grips while trying to tighten/lossen to break them loose with a spoke wrench.
& hit them with a bit of localised heat.
If you have spares cutting them is way easy option. Straight pulls are such a pig when you are rebuilding a pretty out of shape wheel it is just easier to cut them all & start from new as tensioning a spinning spoke loses it’s humour value pretty quick.