Print using Add-in Only (Excel 2010, VBA)

I am trying to disable the PRINT function being used be separate workbooks. The only time I want it to print is using an AddIn. I have attached an example. It shows the messagebox but then allow the Excel Print function to work without using the AddIn. Additionally, the AddIn is getting the messagebox.

Book1 in ThisWorkbook

Public Sub Workbook_BeforePrint(Cancel As Boolean)
    Cancel = PrntOK
    MsgBox "You can't print this workbook"
    Exit Sub
End Sub

AddIn Code (partial)

Public Sub PrintDoc()

    name = UCase(Application.UserName)
    ctrlNbr = "12345"
    PrntOK = True
    Call PrintDocX(name, ctrlNbr)
    PrntOK = False
End Sub