Take a Chair – Sit Down There

We have an owner who has practically tricked everyone (even the CTTT) and enclosed a part of the common property with a fence. He has direct access to the area from a gate he has inserted in his balcony fence.

Now he takes a chair and sits out – it is so obvious that he is making this area his own. Can anything be done about it? Is it legal to take your chair and sit there? What restrictions apply to the use of common property?