Should Not or Cannot?

"Shouldn't" is more appropriate as it implies you are not supposed to eat broad beans. It wouldn't be incorrect to use "can't" in this context but it isn't as clear. A person who says he can't eat broad beans may just mean that he hates the taste.

Do not vs Cannot?

Don´t do it if you can´t. The only difference between the two is that "Don't" may be an imperative, while "Can't" is always related to an object. Don't do that versus "You/He/They" can't do that. While don't can state an object as well (you,we,he,she...) it doesn't have to be specific, but can be a general directive.

Is Cannot one word or two?

"Cannot" (one word) is the most common expansion of the contraction "can't." For example: I cannot do it!