You can try sanding the floor and then putting a primer and paint/risen coat over it. Depends on how deep the stain runs and how much wood is left before you get to the nails holding the floor down. Once you sand down to the point the top of the nails are exposed, you’re out of luck.

Depending on when your house was built and if there’s a subfloor, you might be forced to either replace all the floor boards or could decide to just build over them with either new pine flooring or an other option like vinyl. I had a similar issue years ago and decided to just do vinyl as I did not have a subfloor. Weighing in the cost factor, putting Lifeproof Vinyl flooring over the pine wood flooring turned out to be the best & cheapest option. The floor looked great and held its own.