There is Terminate End event feature in the Activiti Explorer.
But it's absent in the user guide.
In my experiments it works but if raised in the Sub-Process all its parent execution tree is killed.
This behavior conflicts with the BPMN2 specification:
"For a “terminate” End Event, the Sub-Process is abnormally terminated. In case of a multi-instance Sub-Process, only the affected instance is terminated—no other ongoing Sub-Process instances or higher-level Sub-Process or Process instances are affected. "