Are you using the Blackberry App World? If so, you are going to need to create a dynamic licensing model; this involves having the blackberry app world request a key from your webserver when the user downloads the application. Check out this document: http://na.blackberry.com/eng/develop...cense_Flow.pdf
Your server keeps track of the license keys and when they were issued, and whenever you make a request to your server you can submit the license key as a parameter and check to see if it is valid. Or, you could just check it once when the application is launched. If the license key has expired, just respond with an error code to your blackberry application.