The activation seems to be only for updates – I had a look through the code and can’t see anything that is restricted without the license key in the database or without passing verification.
To null it (makes the nag go away if nothing else!)…
Edit the file wpachievements/includes/admin/class-wpachievements-admin-update.php
In the function is_verified which starts around line 36, add this to the top of the function