Can builder cancell agrrement

I booked flat and did agreement in Feb 2013. In agreement they mentioned will get possession in 18 months. After 3 years now work is only 4 slabs completed and sanctioned stories are 13 slabs mean 13 floor. Now the brick work also not started .In between 2 years work was completely stop. Then we went consumer court regarding delaying possession and for our compension. Now builder started work and 1 slab done means only total 4 slabs done and sent demand letter of 65% work almost completed. We have already paid before 2.5 years 55% amount. Then we went to builder architect office and asked about, not agreed completion of demand letter then architect replied it was not sent by him. He said I didn't signed the letter and work is only 25% completed and we sent replied to builder mail office id that we are not happy your demand letter. we are not suppose to pay the demand payment Builder took meeting and said if you not pay agreement demand then we will charged 24 % amount per annum interest and cancelled the flat after 6 months. Please guide us what should we now