Filming without consent = illegal in the Netherlands. There is plenty of precedent, even pre-GDPR, that an infringement of the right to respect for personal privacy in principle constitutes an unlawful act (“onrechtmatige daad”). Slapping you to get you to stop is self defence.

I suggest a media campaign, but don't be alarmed if the result of that is you being called a DYKWIA and possibly worse.