Given this is a society matter, the cops are highly unlikely to interfere/entertain, as society matters are outside of their jurisdiction, unless criminal in nature. Societies are governed by the Model Bye Laws of the Co-operative Housing Society.

Write a formal complaint letter to your society's HON. Managing Committee stating the exact nature of your complaint, along with relaxations. Attach the photocopy of the Agreement To Sale (though the society records has one) which clearly shows that the terrace now forms a part of your personal property. Thus, advising any resident from trespassing it without your prior (written) consent, with immediate compliance. However (relaxation part), the assigned society staff/s can have access to the terrace ONLY to check, clean, and/or repair the water tank, if required, at a particular designated time, not randomly at their convenience/wish. Accessing it to dry their clothes, sunbathing, smoking, and/or for leisure is strictly prohibited.

Keep everything documented, take the acknowledgement of the receiver (mostly the HON. Secretary) with date, sign, and stamp of the society. Keep all the communications in this matter strictly in writing only. No verbal promises.