If you believe one of your customers has created a scheduled task
performing malicious activity on the server (for example, sending out
spam), you can disable all “Run a command” scheduled tasks while you
investigate.
Log in to the Plesk server via ssh and run the following command
plesk bin server_pref -u -crontab-secure-shell "/bin/false"
to set the default shell for scheduled tasks to “/bin/false”. This
will make all “Run a command” scheduled tasks created by customers to
fail.
Note: Customers that have the permission to select a different shell can
override this for their subscriptions by selecting a different shell
in Websites & Domains > Web Hosting Access.