You can stop windmills. However that changes the investment nature of wind farms. There are a lot of ways to structure this, and no one answer so it is hard to type up all the different issues. I'll pick one simple case, but don't assume this applies to all.

An investor (group) gets a contract to supply wind to a utility for $x. This price is pretty low since wind is variable, but it is enough to predict over a year how much they will make and so for the investors it works out.

For the utility they need more green energy in the mix, and so they are willing to sign the contract even though sometimes they will buy electricity and they pay someone to take it on the spot market. They also know how much wind they expect to get, and so those times wind is negative are made up by times that wind is blowing when they would otherwise have to buy much more expensive peak plant power. So for the utility over the course of the year the wind power is profitable even though at some moments it is unprofitable.

The problem is in the above separation the windmill owner is paid for all power produced, and so if they shut down there is no money for them. (or they have to agree with the utility on a fair payment)