My advice will be always end in a good term. You can opt out anytime you want, even after you started with them. That's why you have "probationary" period. It works 2 ways.

Take time to fix an appointment to reject the offer and explain to them why. A good company will understand and the fact that they wanna employ you mean you have something they want. Maybe not this time, but who knows, when you progress you might ended up with this company again.