How to stop people kissing your baby!!!

My partners step dad keeps kissing my son. We have a strict no kissing rule for him, even I don’t kiss him. I have said something twice then the 3rd time asked my partner to (knowing that I will not be as polite the thirds time). He ended up texting his mum saying that I don’t want him to be kissed- bit of a cop out considering he doesn’t want it either but just doesn’t want to upset mumsy so said it’s me lol.

Anyway it happened again and I don’t know what action to take. He will not be staying with them at all to be babysat so that’s one thing I’ve put in place. But I’m breast feeding so that was the case anyway

I feel so unbelievably disrespected. I know it’s not a big deal to some but this is a boundary I am not budging on. What would you do? I’m thinking about refusing to see them On Christmas but im not sure if im being over the top. Any advice? What would you do?