At the moment, Plesk does not allow disabling of mail service through service plans.
As workaround, you can set up event handler (at Tools & Settings → Event Manager) to track events Domain Created , Domain Updated , Website Created , Website Updated and run script, which would determine whether mail service should be disabled on domain and run
mail CLI utility (
%plesk_dir%\\bin\\mail.exe ) to either enable or disable mail service on domain.
For more information about using Plesk's event tracking feature, see chapter Event Tracking in Administrator's Guide for Plesk 11.0