As said, terminating a backend is the current way to kill a job.
An alternative if this is something you do often:
https://github.com/GoSimpleLLC/jpgAgent
jpgAgent supports terminating a job by issuing a NOTIFY command on the
correct channel like this: NOTIFY jpgagent_kill_job, 'job_id_here';
It works well with windows, and all the existing pgAgent admin tools work
to manage it, it's just a drop in replacement for the agent portion.