Provided you were aware of the endorsement when you purchased the puppy and the breeder has it in writing that you agreed, then there is nothing you can do to get the endorsement removed. I endorse all my puppies and will not lift them unless the dog turns out to be worthy of being bred from (in my opinion and to my high standards) and has had all the necessary health checks done, and I feel the owner wants to breed for the right reason and not just because they think it is a money getting game.
Pups cannot be KC registered unless the mother is over twelve months when mated, and under eight years old when the litter is born. Some breeds have their own recommendations such as the bitch being over two years old or having had at least two seasons. This is not a KC requirement but if you are a member of a breed club and it is in their rules then you could be thrown out of the club for not adhering to it.
There are lots of reasons why a breeder might put an endorsment on, health being one, or too many puppies in the breed, an inherited fault or a fault that goes against the breed standard, such as a non standard colour. If people intend to breed they should make it clear to the breeder so they get a puppy suitable for breeding.