Delete data

I have textboxs bound to numerical fields or datetime fields. these fields
are "allow Null".
I find I cannot delete the data if the field had data before.
If I delete the text of the textbox, the textbox will not lost focus.
If I set the causesvalidation to false, the mousepointer might leave the
textbox, but when I update the datatable, the original value of the textbox
return back to the textbox automatically.

How should I do?

Thanks