I am sorry, I did misunderstand you, so here's a more appropriate answer.
If someone does something that annoys or upsets you and apologises, you could say that it was okay, even if it wasn't. That's the polite response, even if it is dishonest. (Certainly it's a very British thing to do and it's something I do without being aware of it)
If, however, you want to be honest rather than polite, then a lot will depend on the circumstances. You used the example of someone bumping into you and apologising. You could say:
"Watch where you're going"
"Be careful" or "Be more careful"
Note that you can add "Please" and "Thank you" to make things less confrontational.
Original (incorrect) answer below...
First of all, apologies if I've misinterpreted your question.
If I'm reading it correctly, you're asking what response you should give if someone accidentally bumps into you (or something similar).
Well, if that is the case then your initial response of "No problem" is correct. You could also say something like "That's fine" or "Don't worry about it".
"Not therefore" doesn't translate into English and isn't correct, I've never heard that said or seen it written down before.
I hope that helps!