Depends on the laws in Belgium (I've no idea what they are)... in the UK for example trespass by itself is not a criminal matter, even if somebody refuses to leave your property... they need to be doing something else.

They weren't trespassing, they were invited!

Aside: why do Americans always talk about trespassing as something that is done to the trespasser? Isn't trespassing the act itself? If I plant myself in your livingroom uninvited, then surely I am trespassing. Why do so many people instead say that I "get trespassed"?