How will users register? If they are registered through your application, you can create a table containing the device identifiers of all registered devices and check activation in the database each time your application starts.
You can have two web services on the back side - register(string deviceID) and checkActive(string deviceID) .
Each time your application starts, in the main method you can call the checkActive method to find out if the device is registered. If not, you can redirect them to the registration screen.
Apple, of course, will prohibit any such attempts, but since you marked it as Java, I assume it is Android or B'Berry, so you should be fine.