Ownership of CAR Parking

Hii There, Yestarday an event happened in our society. During an file check of parking allotement we come to know that Parking no.1 of my Block belongs to my flat and it is in the name of my Dad. We didn't know that earlier, My Dad is the first owner of the flat as per the Agreement. That parking is been used by some third person in the Block for this much years. My Dad inquired in the Central Commitee of society for same and we come to know that books containd the name of my Dad as the Legal owner of Parking. The Society than issued an possession letter to my Dad regarding the same for parking. When my dad approached that person to vacant the parking he told my Dad that he had bought the same from some 3rd person who resided in my house earlier in 2000's & he have that 20rs stamp paper regarding the same. The person is not agreeing to vacant the place. But since the builder have sold the house to my dad as he is the first owner of house & also the books contains the name of my Dad as the legal owner for same what steps can be taken regarding same ...Please suggest.