Sorry seems to be the hardest word...

Occasionally readers actually apologise for bringing their books in overdue, with fines on &c. My normal reaction to people apologising for things is to say "no problem" or "don't worry about it". But that seems wrong in this case - after all, it isn't OK to bring books in overdue, that's why we charge fines.
So what should I respond to such readers? I don't really want to be horrid, because at least in apologising they're recognising their misdemeanour...

[Poll #1338353]