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。
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word-choice verbs
How to say "you may not [verb] here"?
Depending on the strength of the interdiction, you can answer:

  * すしは食べないでください "Please do not eat sushi" (plain negative form + でください) 

  * すしは食べてはいけません "It's forbidden to eat sushi" (て form + は + いけません)
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Some others that haven't been mentioned yet. In first two that contain は, the
は can be omitted depending on context and/or familiarity.

### 〜て（は）ならない － somewhat strong; commanding

>   * [犯罪]{はん・ざい}を[犯]{おか}してはならない → You must not commit any crimes
>   * その[部屋]{へや}に[入]{はい}ってはならない → You cannot enter that room
>

### 〜て（は）だめ － more casual/familiar, but still commanding

>   * ポイ[捨]{す}てしちゃだめだよ！ → Don't litter! (しちゃ = しては)
>   * 一人で行ってはだめ → You must not go by yourself
>   * ここでタバコ[吸]{す}っては **アカン** ！ → You may not smoke here (アカン is Kansai-ben;
> from [明]{あ}かない)
>

**[遠慮]{えん・りょ}（する） － refrain from; a polite way to state a rule or preference**

>   * [車内]{しゃない}での[通話]{つうわ}はご[遠慮]{えんりょ}ください → Please do not make calls inside
> the (train) car / Please refrain from making calls inside the (train) car
>   * [機内]{きない}での[喫煙]{きつえん}はご遠慮[願]{ねが}います → You are requested to refrain from
> smoking inside the airplane.
>

**～[禁止]{きん・し}／[厳禁]{げん・きん} － often used on signs**

>   * [立入禁止]{たち・いり・きん・し} → No entrance
>   * [駐車禁止]{ちゅう・しゃ・きん・し} → No parking
>   * [土足厳禁]{ど・そく・げん・きん} → "No shoes permitted" → Remove your shoes before
> entering
>

### ～こと － used only in writing, such as notices, instructions, etc.

>   *
> [感電]{かん・でん}のおそれがありますので、[手を[触]{ふ}れないこと](https://japanese.stackexchange.com/q/10977/78)
> → At the risk of being shocked, do not touch (Maybe the label on an
> electrical box)
>
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(I'm responding to your afterthoughts)  

> おすしを食べてもいいですか？  
>  Could you use this in response? おすしを食べることができません。  
>

I think you could say:  
お寿司 **は** 食べることができません。  
お寿司を食べること **は** できません。  
お寿司 **は** 食べられません。  
or お寿司 **は** 食べること **は** できません。etc.  
... using the binding particle は somewhere.  

> おすしを食べてもよくないです。  
>  one-- is this grammatically correct?  
>  two-- is this saying what I think it means (you may not eat sushi)?  
>

1-- No  
2-- Hm, if I heard you say that I'd think you meant it... (^_^;)  
I think you could say more like:  
お寿司は食べてはいけません。  
お寿司を食べてはいけません。  
(I think "お寿司 **は** ・・・" would sound more natural as a response to お寿司~~~ですか？)
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