Mine were into concrete, not metposts, but similar. The wet, damp wood expands and jams itself into the hole and I never found any way to get the damn stumps out other than breaking the surrounding concrete.
With metposts, I'd try to pull them out if they're the taper spike type and as suggested, burn the stumps out.

I have slotted concrete posts now. Downside being digging 3 foot deep holes in wet, claggy clay.

Martin