No, not AfterUpdate - too late. Assuming you want to use vba and not table level validation? You don't want to cancel the error as that would be akin to ignoring it. What you want to do is either trap it in the form BeforeUpdate event and cancel the save or validate the control in the control BeforeUpdate event. Some advocate the form; I'm not that fussy about it but think sometimes it's better than waiting until the end because if you Undo against the form, everything that was entered can be lost.
Code:
Private Sub NoInFull_BeforeUpdate(Cancel As Integer)
If Not IsNumeric(Me.NoInFull) Then
msgbox "Some message here"
Me.NoInFull.Undo
Cancel = True
End Sub
I think that will revert the contents if there was any before and leave the focus on the control.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.