Builder has forcibly occupied office and car porch how to take action

A year back I purchased the apartment in Bangalore and has been handed over to me We have in total 16 apartment's and one apartment is for builder and rest 15 are purchased by various peoples and handed over to them , In Bangalore there is a practice to make a makeshift office for builder in the car porch/ground Floor and will be used by builder till the handing over to the association, But in this case builder said its his permanent office and its not handing over to association ,and even he placed his office board, First he said its illegal in the association meeting but after couple of months he registered the same office in his name and said its legal now, As I owns one apartment ,I want to know the legal ways to remove the builder office from the apartment, Builder nominated the president and under secretary and they are also supporting builder and president has business interest with him and under secretary is builders child hood friend. What should I do?