some possible response:
"that's alright"
"nothing to worry about"
"don't mention it"
"everything is okay"
if you think the person has nothing to be guilty about, you can simply say:
"sorry, for what?"
if the person is asking an apology for the 'major' damage he/she did, just say:
"sorry? is that all you can say?"
just remember though that nowadays, many people are feel sorry but few has the courage to utter the word. when someone says 'sorry' to you, just feel free to accept it by heart.
:)