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

I had also the same problem and fix it via Jenkins Console.

Go to "Manage Jenkins" > "Script Console" and run a script:

 Jenkins .instance.getItemByFullName("JobName")
        .getBuildByNumber(JobNumber)
        .finish(hudson.model.Result.ABORTED, new java.io.IOException("Aborting build")); 

You'll have just specify your JobName and JobNumber.

Go to "Manage Jenkins" > "Script Console" to run a script on your server to interrupt the hanging thread.

You can get all the live threads with Thread.getAllStackTraces() and interrupt the one that's hanging.

Thread.getAllStackTraces().keySet().each() {
  t -> if (t.getName()=="YOUR THREAD NAME" ) {   t.interrupt();  }
}

UPDATE:

The above solution using threads may not work on more recent Jenkins versions. To interrupt frozen pipelines refer to this solution (by alexandru-bantiuc) instead and run:

Jenkins.instance.getItemByFullName("JobName")
                .getBuildByNumber(JobNumber)
                .finish(
                        hudson.model.Result.ABORTED,
                        new java.io.IOException("Aborting build")
                );