nothankyou

you are welcome (正式一点 不用客气)

no problem (口语用的比较多 没关系的)

don't worry (没事)

doesn't matter (无所谓的)

no big deal (没什么的)

never mind (没关系)

it's ok (比较随意一点 没事儿~)

可以说用it's nothing不过有点奇怪.

尽量不要用 don't mention it 这说法不地道.