How to get rid of zombie processes?

You can't kill a zombie process the regular way with kill -9 for example as it's already dead.

One way to kill zombie process is by sending SIGCHLD to the parent process telling it to terminate its child processes. This might not work if the parent process wasn't programmed properly. The invocation is kill -s SIGCHLD [parent_pid]

You can also try closing/terminating the parent process. This will make the zombie process a child of init (1) which does periodic cleanups and will at some point clean up the zombie process.