Don't think you can do that -- exactly. Once you give a user access to edit the data in the text box, they will be able to add/delete/change it.
So, try a work-around. Keep your memo field text box locked and create another text box that you allow the user to edit. You should be able to make them look like one large text box on the screen. At whatever point you would normally save the data, concatenate the two -- me.textbox1 = me.textbox1 & " " & me.textbox2.
Randy