It depends on why you need to do this.
You can set up a specific domain, say smtpauth.mycompany.tld in Plesk and add mailboxes to that domain with 0 bytes storage, then tell users they need to use that username and password to send email via authenticated SMTP. Of course the user's pop3 username/password will also work :-(
So this won't help you if you want to pick and choose who can use authenticated SMTP.
If you want to do that, unfortunately the only way currently would be to use firewall rules, and that would require all your authenticated smtp users to have static IPs. (e.g. disable authenticated smtp on port 25, enable Submission port 587, then firewall 587 so only allow specific IPs belonging to those customers you authorise to use authenticated smtp)
I have put in a feaure request asking for the ability to define who can use authenticated smtp or not. I think this would be a great feature to have.