Hi there, i don't want to sound harsh but did you read the official documentation?
http://sourceforge.net/projects/jbpm/files/jBPM%205/jbpm-5.0-CR1/jbpm-5.0-CR1-docs.zip/download
(this is not on the official webpage, but it will be there soon).
You have a Client to interact with the Human Task Service. In that client you can call the abort method for each task that was created from your processes.
Greetings.
PS: we usually use the "Human Task" workitemhandler id to bind the ServiceTaskHandler, did you change the name of the service in the BPMN2 file? did you see an exception like this: "org.drools.WorkItemHandlerNotFoundException: Could not find work item handler for Human Task" in your logs?