Does it give the name of the action? I not, dePBO the addon and look through the config file. Look for anything regarding the fast rope scripts.
There should be a line similar to:
BAS_fastRope = addAction ["Fast Rope Insertion","fastrope.sqs"]
In fact, a good idea would be to press ctrl+f to get up the search box and type in whatever it says on the action menu. In the example I provied before
BAS_FastRope would be a global variable that defines the action. You can then add this to an init.sqs:
Player removeAction BAS_fastRope
If the action isnt in the config search around in the scripts