Illegal Occupancy of Pump Room

Hi, Our society is having 7th Floor and above to that there is terrace. There is one society members is the relative of builder who is owner of 7th Floor Flat. On Terrace area there is one pump room and same has been mentioned on original map submitted to Municipal Authority but builder had given all the right of the flat to his relative, who is staying at there and keeping his furniture at pump room. Our society has been formed on January 2018, after that we had sent a letter to builder relative to vacant Pump room, on that letter he had submitted the builder letter to us which is signed by builder accountant, In this letter if has mentioned that builder has allotted this room to his relative and society does not have any right on it. If society went it to occupied it then builder will file a case on society and recovery the penalty from society. Also there is Car Paring area which builder is not able to sale to anyone till society formation, same parking he has handover to his relative with allotment letter which is drafted after society formation. Society wont allow any one to park car in society, as there is water tank inside the society. Society allowed only two wheeler parking. This relative wont allow anyone to park bike in to this disputed parking areas. Pump Room and Car Parking nowhere mentioned in his relative Purchase agreement, even he has not paid any taxes, stamp duty, registration charges on it. Now society wants to take action on above points. Please suggest.