Okay, I can give a more complete answer to describe how to make a custom shortcut.
The desired feature in this case appears to be under "Transform Modal Map" and the desired value is "Cancel". It is by default mapped to "Any Esc" as shown below. (This is in User Preferences-->Input.)
You can simply scroll down to the bottom of that section and click on "Add New" to create an alternative mapping for Cancel, as shown below where I have mapped it to "Ctrl-Alt-X":
. . . and you've now got a new keyboard shortuct for that function. Obviously save your user preferences if you want it to persist.