"It's OK, don't worry about it". If they continue to do it, then you may say "It's ok, you don't need to apologize" or "Hey, it's OK. You don't need to be so polite".
If they still continue after this, then they probably aren't going to change and they are just really polite. If that's the case, you don't need to respond after each time they say sorry. That's how I would handle it.
"Not a problem"
Would be my best responses