I can only think: who is the arse of a CEO that doesn’t notice this going around or, worse, does accept this nonsense?

Sure, you could set more of a boundary and talk up to the people. Or they could just have common sense on their own. Formally though, your manager is the one who should stop these childish behaviours and make sure that the pillars of the organisation — like office managers — are kept upright and respected.

I don’t think it is a startup issue persé, more of a company culture one (or the lack of such).