Following the suggestions on this site, I bought a 1/4" compression cap for the unused icemaker water line under the sink. After installing it with Teflon tape, water is still spewing out. I noticed that the icemaker line I removed has a male piece that plugs into the supply line and then a female nut to tighten it to the line. Is there a cap that would have such a piece that would go into the line to improve the fit?
Additionally, add insert one of these into the tube after the nut and compression ferrule have been placed on the tube
Tighten the nut onto the couplers body only until the bottoms out or stops turning. Brass nut will crack from over-torquing. Remove the other nut and ferrule from the couplers other end. Wrap (2) turns of teflon tape in a clockwise direction on these threads. Tighten a compression cap using directions as mentioned for nut above.
Compression fittings have a "finer" thread than standard International Pipe Thread (IPT).