Well, how exactly you answer such a letter depends upon what it actually says. It is just a letter, though, and not a court order. If you have not been molesting or pestering your ex you just say so and say that you have no intention of doing so in the future either.
It is in practice very easy to get a non molestation because all it orders is what someone should not be doing in the first place. If, say, an order says, 'A shall not molest B' then it is very easy to get an order like that because A should not be molesting B in the first place.
The essence of any reply is therefore along the lines of A not having the slightest intention of molesting B in any way. If then, B did go to court in order to get such an order then A could refer to the letter and say B should pay the costs of the application because it was unnecessary.