Uninstall it on your PC/Mac is enough. If you did have it registered on your Adobe Account you may need to delete the serial in your account there as well.
Please note that you may buy an upgrade to LR6 only if you also own a previous copy, otherwise you need to buy the full version.
Thanks for that. Yes I know about the upgrade, I've just had LR3 sitting around on the disc and I'm having a clear out! I'm using 4.6 at the moment and I'll upgrade to 6 from that, so I guess I might be doing this again sometime soon.