They booked the hall before we even spoke on the phone. How do I say no now?

You need to tell your mother today that the match is closed, and you must do it before she sends out any cards or tells the extended relatives. The financial loss of a hall advance is real money, but it is a fraction of the cost of untangling a marriage that started with a twenty-minute phone call. When your mother brings up the booking, you can look her in the eye and say that you cannot marry someone whose core goals for the future do not match yours, and that you will help call the hall manager to try to recover what you can of the deposit. Your parents acted out of a desire to secure your future quickly, especially given your age, and they treated the booking as momentum that would force your compliance. Stopping that train means absorbing their initial panic and anger. They will worry about what the neighbours three lanes away will think, and they will tell you that compromises can be made after the wedding. You have to let them be angry without letting them make the final decision. Call the other person directly as well, or send a clear message, so they are not blindsided while their own family prepares for November. Taking the blame for being difficult is the price of keeping your life your own.