Like anything if you are approched. Be polite at first and if they are bothering you just smile and walk away. If the become too persistent a scowl and body language shoul be enough to let them know you are anoying them.

It happens often and usually (90% of the time) it's a woman... So I simply say in exchange for sex I'll help them with their English... To my surprise 5 times in 6 years the answer was "ok" which scared me and I ran away (I don't actually expect a yes I expect a disgusted "you pig!" look as they leave in a huff...) I guess with private language institutes costing upwards of 20,000RMB+ they figure getting laid and free classes maybe a double bonus...

"It happens often and usually (90% of the time) it's a woman... So I simply say in exchange for sex I'll help them with their English... To my surprise 5 times in 6 years the answer was "ok" which scared me and I ran away (I don't actually expect a yes I expect a disgusted "you pig!" look as they leave in a huff...) I guess with private language institutes costing upwards of 20,000RMB+ they figure getting laid and free classes maybe a double bonus... "

Or the plan was to rob you, so getting you to a private place where you'd get jumped was a priority.

That said, next time you hear a yes, I say go for it.

If you have time, talk with them for a few minutes. If they're annoying or you're in a hurry, just smile and politely decline.

Of course you will get tired hearing the exact same question and comments every time you are approached. Of course that may be some that are trying to scam you out of something or want to use you. For the many that approach so timidly and can barely say more than a couple of phrases, remember that it took them a lot of courage to even approach you and spending a few minutes to talk to them would make their entire day.

if you are an american threaten to shoot them because many believe americans are always armed even in china

Americans allways Shoot first and ask questions after, that way they don't get any bullcrap.

1) If they are really friendly, I give them a few moments, as I know it can be hard/lame/unrewarding to try to learn a language while not having many native speakers around, so I really do sympathize.

2) If they start asking me condescending questions, or acting friendly in English but making racist or mocking comments in Chinese to their friends (90%) of the time, I walk. If the timing is right, they'll get the point.

if they want to go for coffee or tea,,,, RUN !!

Pretend you don't speak English.

Start speaking French...or any other language.

Tell them u don't speak English in Chinese... ting bu dong...

a friend of mine was led to a small coffee tea shop and was bilked out of 600 rmb for 2 cups of tea, he said there were about 6 guys blocking the doorway until he paid for the tea, at their price.  if you are approached and dont want to take any risks, just ignore them.  it could be dangerous...

I don't understand your topic. I thought the enthusiasts were people who always find faults with your grammar.

Act confused...distract them with a funny look or a gesture and Escape as fast as you can as if you just robbed a bank! 

Pretend that am Dumb! or speak Chinese or maybe use sign language... if you know what i mean! but hey if its a "pretty single girl" why not get her number...

I've started carrying pamphlets for my school around. That usually shuts em up quick.

I answer them politely with "Excuse me but I am in a hurry."  I say with a frozen smile but firmly.

"Language Enthusiasts"!!! .....hahahahahaha, I love that, spot on too. One of the best descriptions ever.

I randomly speak Spanish in an anger tone or while laughing for no reason after that they leave and if they do not you better.

I always try to be nice and answer thier questions. When I know I made someone's day by just saying hello and giving them a smile, taking a picture with them, or doing something nice for them it also makes my day. I like to have fun with the attention of being a foreigner.