Dear Sir,
You need to sit with the drivers and address their issues once and for all, you may enter into a written settlement with them about the payrise and other benefits and put a clause therein that they will not raise any more demands for 2-3 years till the term of settlement; It is not good to allow them to hold you to ransom every time or else you can appoint a contractor by outsourcing to a contractor who can provide you with the drivers, but he should follow all the labour laws and you have to also enter into a proper contract agreement and if the Contract Labour (R&A) Act is applicable, then register with the authorities and have a licenced contractor.