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The milk should dry up on it's own if the kittens are no longer stimulating her. It can take a few weeks, however, for this to happen. The vet can give her an injection that may help milk production, but unless her glands are very hard to the touch, you are better off letting Mother Nature do things in their own good time. Usually, kittens wean themselves off mom from 6-12 weeks of age, so it's gradual that the milk production lessens. There is a 6 week period where they are eating almost all solid food, and nursing more as a 'snack' or for 'comfort' so the milk supply diminished based on demands made by the kittens. If the kittens left mom before 11 or 12 week of age, you'll need to realize that the entire time frame between the time they actually left and the time she would have naturally dried up has to be covered. She'll do fine with patience.
Let me know if I can help further.