To be fair, under EU-OPS at least:
OPS 1.335
Smoking on board
(a) The commander shall ensure that no person on board is allowed to smoke:
1. whenever deemed necessary in the interest of safety;
2. while the aeroplane is on the ground unless specifically permitted in accordance with procedures defined in the
Operations Manual;
3. outside designated smoking areas, in the aisle(s) and in the toilet(s);
4. in cargo compartments and/or other areas where cargo is carried which is not stored in flame resistant containers or covered by flame resistant canvas; and
5. in those areas of the cabin where oxygen is being supplied.
Smoking is also prohibited where the "No Smoking" signs are on ... so - there is actually nothing in the regs to stop a Captain smoking on the flight deck. There may be something in an Operations Manual, or indeed in workplace Health & Safety regulations (although aviation is often exempt.)