This is a common need and there are a number of ways you could approach this problem. 

One way would be to add a parallel path to your workflow, perhaps from the Start activity or IT Requisition activity.  When you have multiple lines coming out of an activity and more than one has a condition that is true, the workflow will split into parallel paths.   Your new path might go back to the originator with some sort of client event that allows them to cancel. 

When you have two parallel paths, the workflow won’t end until all paths are complete.  Let’s say the originator decides to cancel.  You will need to end the main path and perhaps perform some tasks to note it was cancelled.  After the originator cancels the request, you can stop the other path by using the GotoActivity API call in a server code event:

K2.GotoActivity("SomeActivity");

The GotoActivity has a useful side-effect in that it cancels all parallel paths when it goes to the specified activity.  In this activity you could perform any clean-up work and let the workflow end.

Another common situation is the originator should no longer be allowed to cancel after the main path gets to a certain point.  In this case, you could again use GotoActivity on the main path to cancel the originator’s path.  One word of caution:  avoid overuse of GotoActivity.  If you use this too much, it can make your workflow very difficult for someone else to follow as the logic will no longer be in the lines, but in code.

There are a number of other discussions in the forums on this topic so you might want to look around for other ideas.