You will need to use VB event code to control the activation/deactivation of
the validation list. The code below will do that, but I'm not exactly sure
how to remove the displayed dropdown arrow that appears when A2 is not set
to a number greater than zero... the visible dropdown arrow that is shown
for this condition is not active though and any entry is permitted for it
(data restriction to the list is only active when A2 is a number greater
than zero)...
Private Sub Worksheet_Change(ByVal Target As Range)
Dim V As Range
If Target.Address = "$A$2" Then
With Range("A3")
On Error GoTo CleanUp
Application.EnableEvents = False
.Validation.Delete
If IsNumeric(Target.Value) Then
If Target.Value > 0 Then
.Value = ""
.Validation.Add Type:=xlValidateList, _
Formula1:="Item 1,Item 2,Item 3,etc."
End If
End If
End With
End If
CleanUp:
Application.EnableEvents = True
End Sub
To install this event code, right click the worksheet tab and select View
Code from the popup menu that appears, then copy/paste the above code into
the code window that appeared. That is it, change the value of A2 and then
try entering things in A3.