Question 1: Can customer use the same response file (which I have given him initially) every 30 days, again and again?
Yes, until the activation response expires. Which is what the "Expires" field is all about.
Question 2: Now the customer claims that his computer is broken and want to install on another. I deactivate the key, he installs on different machine and activates again on that machine. But his old machine is still running the Software and every 30 days he is activating it with the response file he have. What to do in this case?
Don't allow him to move computers for X days. Or charge him for a new activation. Or force online activations. It's entirely up to you what you do.