Prohibit Entry to Field by Calculation

[Greg M]

I want to allow/prohibit entry into a field based on data in a related Permissions database. For instance, if the Permissions field says "Allow" I want the user to be able to modify the field. If the Permissions field says "Read-Only" I don't want the user to be able to modify the field. (If possible, is there a way to hide what's in the field if the Permissions field says "Deny"?)