Thats the real question isn't it, You can but to who right.
Nothing will really happen though someone else will just do it.
Smoking is a ancient chinese past time, passed down from Phillip morris and BAT

I have been into hospitals that have no smoking signs in the bathroom, but cans used for ashtrays placed next to the urinals to stop people throwing their butts in them. Go figure.
Probably futile but I would still try to report this to the building manager. Non smoking regulations are slowly starting to get a foothold in China and every little action may help.

Yes, you could report it, maybe to the Building Manager or to Security, but do not hold your breath, nothing will happen. Many buildings and establishments do have posted " NO SMOKING" signs, but that is cosmetic, just to comply with the law. Ja, Ja, have you been to restaurants with the "IGNORED" sign posted everywhere, and see a fellow lit a cigarrete, and the fuyan rushing in with an ashtray?. Instead of telling the person smoking is not allowed, they give him an ashtray.

Maybe you should waste your time on other so popular causes with foreigners, like air pollution from smoke stacks, discharge of waste and pollutants into lakes and rivers, or chasing down pickpockets and fake currency handlers.

One of these days I would not be surprised to find either a restaurant or bar with topless or fully naked waitresses, if they can make money from it, some Chinese entrepeneur will come up with the idea.

i would say it's better...all things considered, that they choose to smoke in the washroom, as apposed to smoking throughout the building.
Most upscale restaurants in malls I have been to, are non smoking, but the guy's washroom is full of ashtrays and people smoking. Better than in the restaurant?

Reporting him won't do anything. People smoke almost anywhere here, at least he isn't blowing it in your face. Even though there are signs, there are no campaigns that I know of that tell people not to smoke, so they're just decoration. On the bright side, you can practice holding your breath, maybe you can improve your endurance.

Reporting them isn't going to fix anything. If you actually want them to stop.... hide in a stall until someone lights up then jump out and make a big scene, the loss of face MIGHT shame them into not smoking in your building again (or they just might make fun of you behind your back).

If you dont like smoking or smoke your in the wrong place country

are you really that big of a tattle tale?

a snitch...

loses all respectability...

grow a pair.

I generally just politely attract their attention and point to the non-smoking sign. Almost every time, they put the cigarette out, but if they don't, it's really not a big deal. Most people in China know the things they do are wrong, and panic if challenged in even the slightest way.

 
Depends how much of an issue it is for you but here’s a solution:
 
Stick your phone just over the door (so that the person can see it) and act as if you are taking a photo/video.

(don’t actually do it cause I don’t think it’s the kinda of stuff you'd want to see or the kind of stuff you'd like your mates/partner to find on my mobile phone/automatically synced on your PC).
 
He’ll be worried that:
 

• A- You report him (as you have evidence)
• B- You show his boss/colleagues and he’ll ‘lose face’
• C- He becomes an overnight internet sensation

 
…..it should do the trick!