This is perhaps not the most direct method to solve this, but I remove the contact control from our layouts and use the dynamic label control on devnet downloads and configure the control for the contacts field. The net effect is a read-only representation of the Contact field on the layout.
Of course adding a contact requires a layout with the contact field or use of import or something like Outlook contact sync.
Basically there isn't a direct way of making the contact field read only. As a work around Allen has suggested using the custom control that can be found in the developers download section of these forums. It will display the data from the contact field, but someone would not be able to edit it. The trouble is that if you take this route you'd have to devise a way of entering contacts, such as importing.
If there is only a single person who needs to add/remove records and everyone else only needs to be able to view the data in the database, then perhaps setting them as browse users would be sufficient.
