(1) goods delivered were faulty in the first place
(2) therefore, you should not be held liable for any subsequent repairs as fault was pre-existing
if he persists further, just ignore him.
you mean there is alternative source of action? if he gets nasty, makes threats, try to make sure you can capture it and submit to the police. this will help in a Restraining Order approach if it really gets too bad. but for a lens....? don't think so.