As jim stated, a zombie process is already dead. It has already exited. Any files it opened have been closed. Any memory it allocated has been freed. Aside from its entry in the process table, its not consuming any resources. So, if you only have a few of them, it's probably not worth killing the parent or rebooting.

Again, as jim stated, zombies are the result of a parent not looking after its children. If the parent's code is under your control, you should improve it with the appropriate wait calls. If it is not, you should file a bug report with the author.

Regards,
Alister