How do I disable a custom field on the Create/Edit Contact form?

I want the Contact to contain custom fields. All work except for a field that I want to make read-only: I don't want the user to have the ability to type anything into this field (it's a key for another system) but it looks like I can only make the field required (*) and limit the character input. How do I either hide the field or make the contents read-only? I still need the field to be present in the Contact record (so I can search for it later).