I am have a party for 16 people and I got a call that 2 couples want to come, but I do not want to invite them?

All you have to say is " I'm sorry, we have finalized things". You owe no one an explanation. If you engage them in your reasoning all it will result in is an argument - them telling you the reasons why they feel this other couple should go, and you with your reasonings. Realistically neither of you will change your view on the situation so keep it VERY simple. If they keep pushing just keep repeating yourself " I'm sorry we have finalized things". Keep it short and sweet and they will get the point.