Is there any way to terminate an Batch Import job from the UI? I think that there are several imports that are each waiting for the other, and I can't stop/cancel any of them.
You should see "Cancel" link for each job which is currently running. Please attach screen capture if you don't. Also please provide version number of software you're running.
There are not Cancel links. This is the mission.do (Good Done Great) private cloud version. 
It looks like you need to upgrade to the latest software version in order to have advantage of using functionality to cancel running jobs.
This was first release in 3.8.8 last November:
https://www.rollbase.com/releases.shtml#3.8.8