If you don't want to eat anymore when you are asked if “vous en voulez encore ?, refuse with : “non merci”
If you want some more, don't help yourself, wait for your hosts to ask “vous en voulez encore ?”, then you can say “oui, merci”, “oui, je veux bien”.
It's the same for the wine.
The only thing you can “do by yourself” is drink water. You can fill your glass of water without being dependent on your host !
But don't forget to ask if “quelqu'un veut de l'eau ?”, “qui veut de l'eau ?”.
Back to je suis invité / I'm invited