As a general rule, you (or your users) shouldn't really be working in tables. Otherwise, why not use Excel!
On a form, as Jebry was saying, right-click the field control representing the table field and choose properties. On the Data tab, set the locked property to 'yes'.
I wouldn't set the Enabled property to 'No' though. Doing so will gray out the field and make it difficult to read, unless this is your intent.