How do I remove a collaborator from the itinerary I sent them?

Go to the itinerary. In the title box, click on the X to the left of their company name in the collaborator section.

The itinerary will be removed from their AXUS itineraries feed.

If you are concerned that the collaborator still has access to the web view link (There is a hyperlink to the itinerary web view in their email invite.)

You can do the following: Copy the itinerary and then delete the original.
 
The copy will have a new link and the previous link will no longer work.