I feel your pain, but I think you are J. going to have to put your foot down and start disciplining her for the pestering. It's not much different than if she was pestering you for a puppy or a horse or a certain toy or a trip to Disneyland. You can tell her maybe some day but not right now, and that's it. You gave her an answer and it may not be the answer she wants, but it is an answer. I have to say that to my daughter all the time (age 4) because she can badger with the best of them and does everything she can to wear M. down with her requests once she has her mind set on something.
My daughter would love a baby sibling too and I know she would make a fantastic big sister, but the truth it, I cannot (or at least, should not) become pregnant again. I had blood clot issues with DD and I will be 40 this year and my doctor already told M. I would be considered high-risk, so DH and I have already decided it's not worth it. But all that is too much to expect DD to understand, so for now I J. tell her I am not having any more and figure she'll be able to better understand why when she is older. I do tell her that I know she would be an awesome big sister and I would love to be able to make her one, but I J. can't, and that's J. the way it is.
Like I said, think about what you would do if she was asking for anything else - as far as I am concerned, constant pestering for something you've already given an answer to merits some kind of consequence if it continues. Especially if she decides to go to someone else and ask them instead after you have said no. DD will try that between M. and her dad and it never works in her favor.