I do so, using an licence Server. The Plugin / The Theme sends an licence-code (md5 on the wordpress-url) each hour to that Server and check whether the licence-code is valid or not (also get some remote Settings, if you have multi-mode-licences (Basic, Standard, Extended) or so) and if it’s not valid, the plugin stops working / the theme just Displays an maintenence-page.
May this is an way for you to do what you are planing to do.