The "Upgrade to Pro" button was still visible (and functional) for me after upgrading. It wasn't until restarting the application that the button disappeared.

On a side note, good job on NZB360, the upgrade message made me smirk. You're the example of what a developer should be, friendly and always striving for the best.
if yuo go out of the settings menu and back in, it goes away, but I agree, my first thought was "it shoudl go away right after purchase"