Is it possible to disable or hide the close button (x at
top right of the window) on a Userform, so the Userform
can only be closed by clicking an okay button

You don't need to hide it as there is a QueryClose event that you can
monitor. For instance

Private Sub UserForm_QueryClose(Cancel As Integer, CloseMode As Integer)
    If Not fValid Then
        Cancel = True
    End If
End Sub

if fValid is set to False, the close button has no effect. You could throw
up a message, or just ignore that action. This also disables the close on
the form control menu.

--
    HTH

    -------

    Bob Phillips
    ... looking out across Poole Harbour to the Purbecks

Private Sub UserForm_QueryClose(Cancel As Integer, CloseMode As Integer)
If CloseMode = 0 Then Cancel = True
End Sub

HTH. Best wishes Harald