I had same issue at the last half hour…

Was not able to delete a zombie build running in my multi-branch pipeline.
Even Server restarts by UI or even from commandline via sudo service jenkins restart
did block the execution… The build was not stoppable… It always reapeared.

Used Version: Jenkins ver 2.150.2

I was very annoyed, but… when looking into the log of the build I found something intersting at the end of the log:

The red marked parts are the “frustrating parts”…
As you can see I always wanted to Abort the build from UI but it did not work…

But there is a hyperlink with text Click here to forcibly terminate running steps…(first green one)
Now I pressed the link…)
After the link execution a message about Still paused appeared with another Link Click
here to forcibily kill entire build (second green one)
After pressing this link also the build finally was hard killed…

So this seems to work without any special plugins (except the multibranch-pipeline build plugin itself).