Why don't you try nested if's?
Example:
Private Sub DisableControls()
Try
If gblDestinationCountry <> "" and gblOriginCountry <> "" then
If gblDestinationCountry <> gblOriginCountry then
txtReference1.Enabled=False
End If
End If
Catch ex As Exception
ScriptManager.RegisterStartupScript(Page, GetType(Page), "Error", "alert('An error occurred!');", True)
End Try
End Sub
<EDIT> Can you tell me in what line do you have an error? Maybe, we can work on that. </EDIT>