Cap icemaker water line under sink-cap leaks
So at the end of tubing that you want to cap you should have a fitting similar to this 
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 &quot;finer&quot; thread than standard International Pipe Thread (IPT).