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 should have claimed the reporters were using AC
Just to clarify. The UK police can assist you in ejecting trespassers, whom you have told to leave your house, in order to "prevent a breach of the peace". They won't arrest or charge trespassers unless they have reason to suspect criminal activity.
In this case the Belgian police might have been justified in escorting the journalists off the premises. But I'm not sure what grounds they had to detain and question them.