The major plot hole is that no one would build such a power generation system without first checking to see whether black holes really do evaporate via Hawking radiation. Why? Because Hawking-radiation evaporation is a very violent process, and you'd want to make sure you could contain the process of "shutting down the power station and letting it explode". So you'd test things out by first making really small mini black holes somewhere out in space and letting them decay. (And then even if it worked, no one would want such a power plant anywhere near them in the first place.)
There are two potential ways to get power out of a black hole:
Take a massive black hole (too massive to evaporate in any reasonable amount of time) and throw stuff ("garbage") at it to create an accretion disk; harvest the light and other forms of radiation coming off the accretion disk.
Create a very small black hole and (starting immediately) feed mass into it fast enough to offset the Hawking radiation, setting up a delicate balance where the black hole's mass stays constant. In this case, you're getting some combination of accretion-based energy and Hawking radiation from the black hole itself.
(This would only work for black holes at least a few million tons in mass, because the Hawking radiation from a very small black hole would probably be intense enough tend to push away the stuff you're trying to feed into it, and even if it wasn't, you would probably have to send the mass in faster than the speed of light, which isn't possible.... Also, the Hawking radiation increases as the black hole gets smaller; for black holes that aren't large enough to destroy a city when you stop feeding them [see below], the transient power output is tremendous, so the processes of feeding the black hole and containing the power output would be rather difficult...)
So what happens when you stop feeding it? A black hole with initial mass of 230,000 kg will decay in about 1 second, releasing energy equivalent to 5 trillion megatons. An initial mass of 1 kg decays in about $10^{-16}$ seconds, releasing about 20 million megatons. A 1-milligram black hole decays in about $10^{-26}$ seconds, releasing about 20 megatons. (See here for formulas. These are crude, and more correct versions might change the results by factors of ten or a hundred, but that won't help you.)
You can see why NIMBY opposition to a mini-black-hole power plant might be pretty strong.
[Responding to the comment:]
Is it possible to slow down feeding the black hole, instead of totally starving it days (or years before planning to shut down the power station?) so that the explosion is not as violent, but gradual and controlled?
No, that will actually make it worse.
The problem is that as the black hole gets smaller (by losing mass-energy to Hawking radiation), it gets hotter and thus radiates more. When it's very small, it radiates with enormous power. A 1 kg black hole would have a power output of $3.6 \times 10^{32}$ watts -- a rate of about $10^{23}$ megatons per second. And bear in mind that you would have to feed it at least that much mass-energy equivalent each second (if that's even possible) to keep it from getting smaller and hotter and more luminous.
Really, the safet way to "shut down" a black hole power station is to shove lots of mass into it, so that the Hawking radiation goes down and the lifetime goes up. If you can boost the mass up to about one hundred billion tons or so, then the Hawking radiation will only amount to about 30 kilowatts and the lifetime will be about 100 billion years, so you can let some other poor bastard (civilization) worry about its ultimate fate.
[Edited to correct calculations and address question in comment]