A user does not have to be logged into PerfectForms to lock an instance. When an instance is opened, it is locked for use by anyone else. If the instance is not submitted or closed you will have to wait until it times out. The default setting for time out is 30 minutes. You should consider giving users the option to simply close the form rather than submitting it. If an instance is opened and the user simply closes the browser window, the instance will remain open until it times out.