Copper supply and return lines (1/2" copper) remain from a removed oil heater. The 3' lines are cracked at the concrete floor level and weep oil when moved. What is the best way to seal these lines at floor level? I have thought of using a wooden or metal plug, solder or epoxy. Since the lines are cracked, crimping them above the floor may not be effective.

First I would suck the oil out of the line or you will be fighting this forever. Then once the oil is gone cut them off at floor level and cement patch over the lines. The cement will flow down into the hollow lines forming a good seal. BUT you have to suck the oil out first.