Good question. I'd be prepared for the worst. In China, it's pretty common for people to completely ignore signs like no-smoking. Smoking is a favorite activity here for many and a integral part of life of a large portion. China has probably one of the highest smoking rates for males and females compared to almost any nation.

You can tell them and hope they move, but I wouldn't be surprised if they simply ignore you. Your best guarantee is to get up and move.

As for what to say, I'd be a little more forceful in what you say if they are smoking in a non-smoking area. Saying words like "please" only shouts out that your are a tourist too. Hence, I'd say:

Bu yao xi yan 不要吸烟. That's when I would point to the sign. If they continue to ignore you, you can escalate it to a waiter who should comply. The secret is to insist and make a big enough of a fuss. In general, Chinese fear making trouble out of things so at this point they will not smoke. Or if it is simpler for you to move, then just move.

BTW, xi yan and chou yan both mean smoking in China, but depending on where you are one is more commonly used than another. In PVG xi yan is more commonly used in public places like restaurants. I think the same is for PEK. Either is ok and should be understood.

Smoking is one of my least favorite parts of China as I'm anti-second hand smoke. But alas, it's a part of being in China and there's an element of learning to accept vs fight.