please leave now, we are done here - BUT with the most polite intent.
It is the same as saying, I would love to take the time to explain further, or at least make up a good reason why you should now leave, but I am too busy to do either, please let me do my work.
When someone is in your office and you answered their question, but they are still there and you need to get work done, simply say "phishwa". You can also give them an upside wave as if scooting them along. Works well with kids too.