This is a complicated question.
On the one hand, it does seem that smokers should be barred from using the government health system. They are choosing to take an action that is almost certain to be hazardous to their health. It is an action that has no possible benefit (which could separate it from things like people who hurt themselves doing things like playing sports). Therefore, we can clearly argue that they should not be allowed to use the healthcare system, thus imposing costs on other people.
On the other hand, this seems like a bad idea for two reasons. First, it is somewhat inhumane to ban a person from getting the healthcare they need, even if they have made bad choices in their lives. Second, it seems that it is unfair to smokers to single them out. It is not clear that their choices are any worse than the choices of a person who overeats and becomes obese or a person who fails to exercise and thus ruins their overall health. If smokers are to be banned, why not these people?
Perhaps one possible compromise would be to charge smokers (and possibly others, such as obese people) more for any procedures they must undergo that are clearly connected to their bad behavior.