I have a split water line that went to two refridgerators. After replacing one of the refrigerators, I no longer need a water line going to that refrigerator. How do I stop the water from flowing in that copper tubing?

You are likely talking about the 1/8 flexible copper tubing. There should be a shut off or a saddle shut off near the beginning of the line. Look for that before we try anything more complicated. Back to you.

Several things come to mind. (1) change out the fitting that splits the lines to one that directs the water to the icemaker, (Best)
(2) Crimp the end of the unused line and solder it shut.
(3) Shut the splitter fitting off and install a saddle valve on the hot water supply and connect the icemaker line in use to it.
Your choice. Good luck, Tom