A System Parameter has been entered to the Booking Process Parameters that allows you to choose whether notes can be entered on the Booking Form or not. 

This controls the notes that are entered at the bottom of the Booking Details tab in the Booking form. If the Parameter is set to "Yes" it will appear as shown below, and you will be able to enter, update and delete notes here.

 

If it is set to "No" and there are no pre-existing notes against this booking then this notes field will not appear on the Booking Form at all.

If it is set to "No", but there are pre-existing notes against this booking, like those entered by a customer through your website, then this field will appear with the notes, but you will be unable to edit them.