Hi Jay -
In your MPX Administration web page, go to User Group Management. If you have not created special ones, you can edit the default System. The Meeting Preferences field, "Host web meetings with" can be coded as Participant list only (no licenses required). In this way, web ports will not be used by the users associated with this Group. You may consider creating a new Group that modifies just this setting and apply your users that only require voice ports.
Regards, Ginger