In the approach phase, engine 1 of [Cathay Pacific flight 780](https://en.wikipedia.org/wiki/Cathay_Pacific_Flight_780) got stuck at about 70% N1 and it forced the crew to do an overspeed landing (230knots).

Why didn't they shut it off by turning the fuel pumps off? Is there a backup mechanism if the fuel valve fails like in this scenario?
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Engine #2 wasn't doing its job either
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Had the situation just been engine #1 stuck at high thrust, with engine 2 normally controllable, than what you describe would be a reasonable response to the situation. However, that was *not* the case with CX780 -- during approach, Engine #2 was stuck at 17% N1 (or rather below idle) and thus delivering effectively nil thrust.

As a result, the pilots dared not shut down engine #1 until they were safely stopped on the ground.
Aside from what Unrec. and GdD point out, even if the other engine had been running perfectly... 

A frequent problem with twin engine airplanes is **[shutting down](https://en.wikipedia.org/wiki/Kegworth_air_disaster) the [wrong](https://en.wikipedia.org/wiki/TransAsia_Airways_Flight_235) engine**. Nobody ever expects or plans to shut down the wrong engine, but it happens anyway. 

So a crew is very cautious to shutdown an engine that is working.