Prevent user from deleting the value in a given cell

Hi,
I have a column with dates. I NEED that column to have data. By default, the column will always have a predetermined value.

I need to allow the user to change the values, but do NOT delete the value (Cell must NEVER be empty).

How can I do that?

Thank you very much for the help.

You can use a Change Event and ensure entry is valid

Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target,Range("A1:A100")) Is Nothing Then Exit Sub
On Error GoTo ExitPoint
Application.EnableEvents = False
If Not IsDate(Target(1)) Then
    Application.Undo
    MsgBox "Computer Says No",vbCritical,"Action Reversed"
End If
ExitPoint:
Application.EnableEvents = True
End Sub

to insert the abvoe right click on tab of interest - select View Code and paste above into resulting window

(modify ranges and logic to suit)