You can change the behavior of the field to not allow entry into the field in browse mode.
Another way is to go into the layout and whever you want your fieds, let's call them 'field_a' and 'field_b', you can use the text tool and place the fields as read only by using double carrots.
They will be read only. Not enterable in browse mode.
Now that is cool! Much nicer than a read-only field, because users won't be tempted to enter text, which would bring up the "Cannot be modified" dialog and annoy them. You can, of course, use fields from a related table by using the RelatedTable::field_1 syntax within the carets. The easiest way I can see of doing this is by using the Insert--> Merge Field... menu item (Command - Option M on a Mac), which does not require you to remember how to spell anything.
Also known as Insert > Merge Field.
Doing what John suggested (disabling Browse-mode access) does not cause any error dialogs. You may be thinking of calculated fields.
You are right. I used (up until now) calculated fields to restrict access selectively. They let you change values on one layout using the direct field, but not on others where you use a calculated one.
Thanks for giving me the opportunity to clarify.