You must implead (sue) the person as a party and ask for a stay order against the illegal construction. This can be done after proving your ownership before the court with the relevant property documents.
You can also file a civil suit for an obtaining an "injunction" against the person trespassing and continuing illegal construction, under Section 37 of the Specific Relief Act, 1963. An injunction is a court order that will compel a party to do or refrain from specific acts. This can be done if you feel that compensation is not an adequate relief. For this, you will need to prove to the court that "irreparable harm" will be caused if the injunction is not granted. You will also need to prove ownership before the court with the relevant property documents.
A permanent injunction will put an end to the illegal construction, following which the court might order demolition of the construction and return the property.
A temporary injunction can also be asked for, which will continue until a specified time stopping the encroachment till the suit is disposed off; while a permanent injunction is given at the end of the suit to stop the encroachment which has been undertaken without your consent.