In my opinion, the word 'bak' in the sentence seems to be strange. In fact, the word 'bak' (the synonyms are 'bagaikan' or 'seperti') means 'like' in English.
Nevertheless, based on the context, I believe that
bak = only
Thus, my understanding of the sentence is that in the past there many smokers, including men and women. Nowadays, after the implementation of law prohibiting smoking, the number of smokers decreases and even they are only men. There are no more women in the smoking group.

As Hipyan has pointed out, ‘bak’ normally means ‘like’. I am a great believer in human fallibility however and I would guess that the writer was trying to say that smokers in the US are now mostly men. Possibly it started out as ‘kebanyakan’ (mostly/mainly) and with a few injudicious taps of the keyboard, hey presto it became ‘bak’. Reporters work under time constraints and so do editors, hence mistakes are not uncommon. This seems more plausible than ‘only’ as there were nearly 20 million women smokers in the US in 2007 (according to the American Lung Association).