I agree with Thiggy that it's preferable not to leave abandoned supply lines in the walls - just be real careful capping anything in the basement - make sure these lines don't continue anywhere else in the house.
You can cut back the iron pipe with a Sawzall and a special blade (Home Depot/Lowes will carry them) and then cap it with a Fernco type cap with an adjustable metal band to keep it on tight.