I assume you have fmp 12. Your title dosen't match your question. To answer your title: There is not a way to disable a field, there are work around such as having a layout with normal fields for data entry and using merge fields on a layout just to view the data.
To answer your other question.
Select the field you want to change in layout mode then go to the Inspector and click the "Appearance" tab, then At the top of the inspector under "Style Cutom" select the four button (the rectangle icon with a slash thru it) which is the remove all sytle button then add your boarder back by selecting "Solid" under Line.
I tried no style by following the step you shared with me here and change my font Arial with 18 pt.
as long as I select the field, I found the big black background highlight cover the whole field.
seem not to work at all.
anything that I didn't do right?