Here's a fun topic guaranteed to lead to heated argument: Should the World Health Organisation be allowed to refuse to hire smokers? The Financial Times this weekend reported that the WHO was taking its ideal to set a good example a step further by adopting a policy of not hiring smokers in the future. The ban will, naturally (?), not apply to current staff (who are, btw, given assistance if they wish to quit the habit).
There are many hurdles here even without considering purely legal constraints (although, as the FT article remarked, it would seem that anti-discrimination provisions, both in treaties and in national law, generally do not protect smokers specifically). I would tend towards the view that although the objective is positive, its enforcement will trigger privacy concerns of such magnitude that they would outweigh possible benefits.
One problem is the fact that with smoking, the (public) activity - which importantly assumes an expressive dimension in the face of opposition - and underlying (private) choices cannot really be separated. Surely it should be possible to ban WHO staff working in the relevant policy field from publicly defending or encouraging smoking. Similarly, it should be possible to forbid them from publicly defending unsafe intercourse - another activity that is perfectly legal in itself. In the case of unsafe sex, the privacy concerns involved in regulating the actual conduct are immediately obvious (also: the activity is more directly related to existing discrimination safeguards). The smoking case occupies a less akward position on this sliding scale, but I wouldn't be sure whether the difference is big enough.
And on a completely different note: Wishing you a good Sinterklaas evening from The Netherlands!