Turn off the supply and drain it down? Same as you do with any leak?
The tank should have been covered so this couldn't happen, and the overflow big enough to take the full output of the rising main. Normal overflows frequently aren't - but can only cope with a dribbling valve.
There were so many flaws in that installation you can't really blame the principle but the cowboys that installed it. You might as well blame all combis for blowing up and killing people - or poisoning them.
Heh heh - but China like all other countries make good and bad products. It's up to the seller to make sure they are fit for the purpose.