たとえば、おすしを食べてもいいですか？ is for "May I eat sushi?". However I know that ちょっと、食べなくてもいいです。Is not the right phrase because that means: "Well, you don't have to eat (that)." So what is the response for "[Sorry,] you may not eat sushi."?
Afterthoughts (after reading istraci, chocolate, and the two "users" responses).
Could you use this in response? おすしを食べることができません。
Also, one is this grammatically correct and two, is this saying what I think it means (you may not eat sushi)? おすしを食べてもよくないです。