Third party employer is not letting me join client company

Hi..I have been working in a company in pune through third party.but that third party was not doing any proper work like pf related work etc and he used to do salary late even company used to give him salary on time.so our company suggested us to leave that one and join them through another one but after resignation the third party employer told us that he has updated hr policy about we can't join client company for next 1 year without his permission.although we haven't signed any of the documents and now we need relieving and experience letter from him to join our company through other third party he is forcing us to sign that document and blackmailing us that if we don't sign that he will not give any of the documents.please help