In the [operation panel]->[Application Definitions]->[Expiry] dialog box, you can check the "Enable expiry" option to protect your application.
When the application is expired, a dialog box will appear and tell the users the application is expired. If the "enable unlock" option is checked, the dialog box will show a "Register" button. The users can click the button to finish the registration. The registration codes can be generated by the "Serial Number creator" in SWFKit.
In SWFKit 3, you can do all these things in your own way. A new object "Application.Expiry" is provided.