As the web search results show, processes in state Z are defunct or zombie processes, which means they have terminated but their parent process has not reaped them properly. They do not consume any resources, but they occupy a slot in the process table. To remove them from the process list, the administrator needs to kill the parent process of the zombies, which will cause them to be reaped by the init process (PID 1). Killing the zombies themselves or the init process will not have any effect, as they are already dead. Rebooting the server may work, but it is not a safe or efficient option, as it may cause unnecessary downtime or data loss for a mission-critical server.
Reference
Processes in a Zombie (Z) or Defunct State | Support | SUSE, paragraph 3 linux - Zombie vs Defunct processes? - Stack Overflow, answer by admirableadmin How To Kill Zombie Processes on Linux | Linux Journal, paragraph 4