[jenkins] How to stop an unstoppable zombie job on Jenkins without restarting the server?

In case you got a Multibranch Pipeline-job (and you are a Jenkins-admin), use in the Jenkins Script Console this script:

Jenkins.instance
.getItemByFullName("<JOB NAME>")
.getBranch("<BRANCH NAME>")
.getBuildByNumber(<BUILD NUMBER>)
.finish(hudson.model.Result.ABORTED, new java.io.IOException("Aborting build"));

From https://issues.jenkins-ci.org/browse/JENKINS-43020

If you aren't sure what the full name (path) of the job is, you may use the following snippet to list the full name of all items:

  Jenkins.instance.getAllItems(AbstractItem.class).each {
    println(it.fullName)
  };

From https://support.cloudbees.com/hc/en-us/articles/226941767-Groovy-to-list-all-jobs