You might have to check your statements & see when the Pro payment was made, or you could always try downloading a newer Pro here...
http://sales.limewire.com/support/pro_lookup.php
...if it works, you'll know that your 6 months aren't up yet

Sorry, but I don't think there's any way of getting rid of an update prompt, apart from updating. I'm a bit confused actually, I've never seen an update prompt from any of my Pros...if it's an upgrade prompt you're seeing then you'd be using the basic version. That prompts you to upgrade to Pro.