Asks for password for protected VBA code when closing Excel

Hi, there
I have a spreadsheet with a piece of VBA code built in at "ThisWorkbook" as below:

Private Sub Workbook_Open()
intbutton = MsgBox("If you wish do A Press 'Yes'. To Do B 
 press 'No'", vbYesNo, "Something")

If intbutton = vbYes Then
'Do A
End If

End Sub

Additionally, there are some forms with built in VBA codes. All the VBA codes are protected with the same password. Here is the problem, after close this speadsheet, if it is attempted to close Excel after it, a message comes out and asks for password for the VBA code. A several methods were tried to get rid of this problem. It seems it only works if do the following step

Close the spreadsheet
Open Microsoft Visual basic editor
Highlight the VBA project of the speadsheet
Close Microsoft Visual basic editor
Close Excel.

Does anybody have even met this problem before? How can I solve it without going through the step listed above?

Thanks in advance. Your time and effort will be highly appreciate.

Best regards,
Chunli