That will depend on the situation, but a few possible answers are: No, do not worry (positive). It is ok (positive) That is fine (positive). Do not worry about it (positive). Do not be (meaning "do not be sorry") (positive). The harm is done, no use in being sorry now (less positive, somewhat negative). I wish you had not done/said it, but you already did (somewhat negative). I do not want you to be sorry, I would prefer you did not do/say that (somewhat negative). There is a lot more, but I think that gives you an idea of what you can say.

Australian English: "No worries!" :) People often say "it's ok" or "it's fine" too. That's assuming you are forgiving them.