The analogy of ex-smoker and alcoholic
mentioned by whizkid
is not entirely correct, since alcoholism is technically a disease
, whereas smoking is an addiction
. 'Drinker' is analogous to 'smoker'; perhaps 'ex-smoker' should be analogous to 'alcoholic who does not drink'.
'Heroinism' is defined as 'an addiction to heroin', and so could be placed in the same category as an addiction to nicotine (although 'nicotinism' refers to nicotine poisoning). However, in this case I believe that the differing terms are due to the usual danger to health in heroin withdrawal symptoms; if nicotine withdrawal were as serious, you might hear more people saying that they had a 'nicotine problem' or a 'cigarette problem'.
I still agree with the overall point of whizkid's writeup however; admitting that you were once a smoker and always will be can be helpful, despite the unconventional terminology.