You can disable the POP/IMAP ports as a system administrator, though this will affect all users of the mail server. Therefore, only do this if you want ALL users on SmarterMail to only use webmail.
Alternatively, you can restrict individual domains by disallowing POP/IMAP services on the Service Access tab for the domain. This is generally handled by logging in as the Domain Administrator (or clicking Manage when you're logged in as the System Admin) and going to Settings > Advanced Settings > Default User Settings, making that change on the Service Access tab, then propagating the change to all users of the domain. You can use this method for all domains on the server as well as a System Admin by modifying the User Defaults, then propagating that change to all domains on the server.
I hope that helps.