You shut down a wind plant by feathering the turbine blades so they don't cause rotational forces. Reasons for doing this would be that you have excess power or if the wind plant is causing some sort of power quality problem. The over generation problem can get interesting. For example, in Washington state some years back, Bonneville Power Administration was fined a lot of money by the EPA because a wind plant wouldn't curtail when they told them too, so they had to stop generating with hydro power, but since it was the spring and the reservoirs were full, they had to spill the water which caused harmful nitrogen levels to the salmon fry. So wind ended up killing fish that day.