Hey,
This has to be implemented by yourself. Like Charlie explained, it can’t be forced, it has to be voluntarily accomplished by a user.
If you are afraid of having users using a single license on many websites against the rules, you could code a simple API like WooThemes do - they control which license key is assigned to what website. If a user wants to move a license key to another WordPress installation, they have to log in to their WooThemes account and revoke existing website from the license key (in other words, free it up). Hope this makes sense 
I’ve created a basic API for license validation for my only plugin on CodeCanyon with an idea to extend it later as described above.