If the delay is on the part of the builder, even the booking is under subvention scheme, you may opt for cancelling the booking blaming the builder for this decision.
You can mention that you were pushed to take this decision since the builder did not keep up his commitment that was originally assured when the booking was made in the initial stage.
You should communicate your decision to cancel the booking in writing by sending the same by registered post with acknowledgment.
Let the builder come out with his explanation or rules refusing to accept your demand to refund the amount in full, you can drag him to consumer forum for relief and remedy besides a complaint with RERA