Possible To Prevent Highlighting Text Within A Text Field?

I am using a calendar control based on the one at Brainjar.com. I have made the text field into which the date is inserted readonly. The problem with this is that if a user attempts to change the date manually by deleting part of the highlighted text, the browser goes back one page.

I want to force the user to use the calendar control and not input text directly into the textfield- readonly is not enough as the text can still be highlighted and then causes problems if the user attempts to delete.