You can stop VPOP3 by going to the Services applet in Windows, find the VPOP3 Email Server service and stop it, or, if you have the VPOP3 status monitor running with appropriate permissions, you can right-click that and choose 'Shutdown Server'

You can find the Windows Services applet by several means:

• Right-click My Computer, choose Manage, go to Services and Applications → Services
• Control Panel → Admin Tools → Services
• Start → Run → services.msc