If you aren't doing so already, I'd suggest you also document the problem. Note down the date, time and nature of the annoyance in a log. You can present it to realty office to give them a better indication of what you're dealing with and it strengthens your case. You'll probably have to ask the smokers themselves to stop smoking outside the stairwell yourself first before you can escalate.

You might also want to ask around to see if anyone else is also suffering from the same problem. If more tenants complain, the realty office will likely give the issue more serious consideration. The realty office may also be able to do more if they know which tenants are smoking outside the stairwell. If they know who, they can address them directly and even levy sanctions against them if the problem persists.

Ultimately, it's very hard to get people to change their habits, even if those habits are causing serious problems for others. Depending on the kind of people you're dealing with, this may very well be very difficult to deal with.