To carry out the business of money lending one needs to be a registered money lender and most states have laws for this. Without such registration if one chooses to carry out such business then one shall be penally liable and all business transactions will be held non enforceable in a court of law because such business has been done in violation of state law. In Assam the law is Assam Money Lenders Act, 1934.
Besides there are also RBI guidelines where interest cannot be charged beyond a certain limit and such interest is not enforceable in a court of law. The clauses in the agreement stating such high interest rates are voidable at your option and you may chose to disregard them.
However what worries me in your situation is that you have given them two blank cheques. If the cheques have a date then you are secured because according to RBI guidelines the validity of a cheque is only for 3 months. However if they were undated as well then they might put an amount and try to encash it. If you block the cheques you might be prosecuted for a voluntary cheque bouncing under section 138 of the Negotiable Instruments act, 1881.
So I suggest you file an FIR against the Afghan Citizen and in your FIR mention the details. After such FIR is registered contact the local bank and furnish a copy of the FIR and based on such FIR block the cheques.
That should protect you in future.