It's arguably in the European Convention for Human Rights. Article 8 – Right to respect for private and family life.

This is what Wiki says on the definition of "private life":

For better understanding of perception of Private Life case law should be analyzed. In Niemietz v. Germany case the Court held that: "[It] does not consider it possible or necessary to attempt an exhaustive definition of the notion of "private life". However, it would be too restrictive to limit the notion to an "inner circle" in which the individual may live his own personal life as he choose and to exclude therefrom entirely the outside world not encompassed within that circle. Respect for Private life must also compromise to a certain degree the right to establish and develop relationship and develop relationship with other human beings."

This is why people are confused about your reaction. You shouldn't even be entertaining their demands! What will he do if you don't pay the 10 €? Apply to a civil court to evict you and have the flat empty for weeks/months looking for new tenants?

You should be laughing in their face!