If someone's windmilling against you IRL, they've just given you range control, line control, and pace control. They need to maintain the same pace, range, and line to continue windmilling, and windmilling does not control the opponent on any one of those.

Use any of line (ex: sidestepping), pace (ex: getting in a quick, calculated shot), or range (ex: stepping inside of their arms and using elbow strikes, stomps, or grappling of some sort), to counter, whatever works best for you.

Or use them all to position them how you like, as was done in this gif, in a game where windmilling works significantly better than in real life.

If they break windmilling whenever it's not advantageous to them, technically it's not windmilling anymore. (Might be rushing, context depending).