how can I train myself not to burst out laughing when I do something really silly?

there is somebody really annoying I have to see most days who doesn't know what boundaries are: always physically too close, always asking prying questions.

because at this point I have nothing to lose, next time he starts his spiel I'm going to start doing really ridiculous stuff, so maybe now he starts avoiding me. Think of:

answering his questions, but only using the syllable la, like lalala lalalalala lalalalalalala... pointing fingers and opening and closing my eyes.

gesticulating and waving my arms like I was possessed by the demon while barking like a dog.

describing in great detail how I like to have sex with several kinds of animals.

problem is, I laugh very easily when I do silly stuff like this and if I burst out laughing while doing it to him, he will think I was doing it to amuse him and keep approaching me.

so, how do I train not to laugh while doing really silly stuff?