2 ways to go, an angle grinder and a masonry wheel or a sawzall blade
with a grit edge.
As always protect your eyeballs.
JA wrote:
We're remodelling a basement and need to remove hard cut (masonry) nails
that are along side a window. There is no way to reach the nails with a
hammer, so pulling them out or pounding them flush is not an option. (They
are in the concrete block wall around a window frame and we need to remove
them to slide sheetrock or paneling into the space.)

Are there saw blades that will cut through them?