How would they determine where the transaction originates?
Maybe, by web wallets from which the transaction is coming? I guess a lot of bitcoiners are using the Coinbase web wallet (I use it too, for the record) which is the US company, so the Chinese miners may just start rejecting all transactions that are coming from this wallet. I also suspect that a few other notable web wallets (Xapo anyone?) are also primarily the US companies...
There may also be American exchanges and mixers which could be heavily hit by this
All Exchanges keep track of your Name / Email Address /IP Address , even if you use a fake name and email, using cross reference techniques your location can be determined by your IP alone.
All China has to do is get an approved list of BTC address from the Chinese Exchanges and just block the rest.
EASY. 