1. How to suspend smtp services for selected users. Or i need to suspend all users smtp clients and need to enable only selected users for smtp services.
you have some domain hosted at Plesk and this domain has enabled mail service and there are some mailboxes that are created at this domain.
And you need to disable an ability to send email from all those mailboxes except some of them (inside this domain).
if yes - unfortunately there's no such functionality in Plesk.
Did you see the link that I send you? mail: Mail Accounts
Here you can see al the possibilities
If there's no way with plesk you can use while:
plesk bin mail -l | grep "Mail name" | grep "example.com" | awk '{print $3}' | while read a; do plesk bin mail --update $a -outgoing-messages-mbox-limit 0; done