Csdrhrt

I don't know how to differentiate between these 2 meanings of 控える:

5. 〔止める，見合わせる〕
   勝手な発言は控えてください
   
   Please refrain from speaking without permission.




6. 〔少なめにする〕塩分を控えなさい
   
   Cut down on your intake of salt.

In this sentence:

少なくともそれまではお酒とたばこは控えてもらえますか？

How do we know if it's limiting or stopping completely?

控える basically means cutting or refraining to the extent reasonably possible rather than stopping completely. In the case of 塩分, it's impossible to completely remove salt from food in the first place. In the case of 少なくともそれまではお酒とたばこは控える, most people would probably try to stop them completely, but some people may just abstain from them. If a physician seriously wants a patient to stop smoking completely, he would unambiguously say 禁煙してください or タバコは止めてください.

In your case, the actually implied prohibition level is still not clear without broader context. But if I'm allowed to guess from that piece, I tend to think you are advised to stop.

Generally speaking, 控える is a polite way to tell that something is strongly discouraged, i.e. the less the better, but you might do it in the degree it doesn't get much in the way of the goal. This verb has another meaning "wait on someone at the side", so you can see that you are expected to do nothing, or at least act without being obstructive. For example, imagine if the chair says 勝手な発言は控えてください, the entire conference room is their earshot, so it practically equals "don't do".

PS

According to 大辞泉:

２

㋐度を越さないように、分量・度数などを少なめにおさえる。節制する。「酒を―・える」「塩分を―・える」

㋑自制や配慮をして、それをやめておく。見合わせる。「外出を―・える」「発言を―・える」

[補説]2㋐㋑について、「抜歯後はお酒を控えてください」とあった場合、㋐の「少なければ飲んでもいい」の意味ではなく、㋑の「自制して飲まない」の意味ととらえるのが妥当であろう。「アルコールを摂取した場合は運転を控えてください」は、明らかに㋑の意味である。

In my experience, 控える appears in medical advices is "never" in most cases. If they want to tell you to reduce the amount, I feel they will say 控えめにしてください.