There is no straight forward option for this.
You can create a simple flex application and allow requesters to terminate their calls.
Here is just a though.
On the Properites of the "Assign Task" of the User Service > Route Evaluation > create two routes: "Delete" and "Forward"
then at the "Initial User Selection" > select the option "Assign to process creator" this will allow the form to go back to form filler (which is now an Initiator) to have an option to verify the form and decide whether to forward to continue the process or delete the form to cancel the submission by click on either button "Delete" and "Forward" button.
Hope this will hope.