in Form design select the textbox.... then on the menubar select View..properties and in the properties dialog screen that comes up scroll till you see the 'enabled' property and switch it to read 'No'
Jim :)
Jim's answer is correct, I just wish to add to it.
If you want the selection to be visible but greyed out, then that is fine.
If you set Lock to yes, then Enabled to know it is a display field only as well, but will not be greyed out.
Finally, If you set Locked to yes and leave the control enabled, the box will be locked for inputting entry, but you can still highlight the data for copying to clipboard.