How to stop breast milk production
While some women's milk never comes in, most will to some extent. You can't really stop it from happening, but you can stop production after it comes in by simply not nursing/pumping. The breast is all about supply and demand. If the baby (or pump) empties the breast that signals the breast to make more. That's why babies nurse more during growth spurts and get fussy. They keep emptying the breast before it has a chance to make more. Eventually the breast figures it out and ups the calories in the milk or produces more.
The reverse is true too. If the baby is nursing less and the breast isn't getting emptied eventually it will figure out that it's making too much. You'll usually feel really full and swollen. It can be painful but eventually the breast will realize that it needs to stop and the milk will dissipate. It's important not to stop nursing/pumping all at once because all that backed up milk can cause engorgement which can lead to a breast infection. It's best to take away once nursing/pumping session at a time to slowly reduce the supply.