Has the mother stopped feeding the litter or has the litter passed?
How long ago was she in labor?
Weaning puppy's should take place at around 4-5 weeks so at this point additional feedings are not only bad for the puppy but painful for mom. Add to the fact that the longer she nurses a puppy the longer it will take for her to recuperate.
The safest and fastest way is to remove the pup from mom for a few days and she will dry up naturally. You can do this by keeping them separate for a few days unless you are there to supervise them. You can also use a large sweater on your dog to cover the mammary glands. You can also use a ace bandage as well. This will help block the area from the puppy if it is hard to keep them separate.
Increased exercise helps as well.