"Can you not" is incorrect English. You can use "Can't you ... ".
e.g. : Can't you refrain from ( / go without / avoid / stop) smoking for three days ?
If you need to use Can, you need to use it this way :
e.g. "Can you refrain from ( / go without / avoid / stop) smoking for three days ?"
the difference between the two is "can you" is asked as a request where as "can't you" is more authoritative (and has a more assertive tone) than "can you". Though both have the same meaning.
A third way to say the same thing is as follows :
"You can stop smoking for three days. Can't you ?"
this means the same thing but has a slightly positive tone of believing that the person can stop smoking.