When cancelling a render, the background render tasks do not terminate, but rather they complete the render. Whilst this isn't so much of an issue for quick render tests, we have trialled complicated scenes that can take a couple of hours or longer to render per frame. This issue here is stopping the background processes immediately.
Please note for animations this is less of a concern since once the background processes finish after cancelling, no further frames will render and cancel works as expected.
Should your frames take a long time to render and you wish to cancel, you can use task manager (windows) or activity monitor (mac) to locate the background render tasks on your two machines and end the processes manually. This has no bad effects on crowdrender, however remember to cancel the render using esc first and then stop the processes. You can tell which ones are rendering since their CPU usage will be pretty high!
When will it get fixed?
This bug is in the next sprint so it should be a couple of months away. Vote up with a like on this post if you think its critical and comment below if you have noticed anything related to this you think needs addressing in the same issue.