I would prefer for my cat to not see her old owner anymore.
What is your reason for preventing your cat from seeing their old owner?
I think they may hurt her.
Just try to understand that the old owner deserves to see their cat as well and would be hurt if they couldn't see them. If you have reason to believe that the owner is going to hurt the cat, then it would be good to keep the cat away from them, otherwise, there is no reason to be worried, so you should let the old owner see their cat.
I know what I'm doing. I'm a good owner and I need to do this.
Well it is your decision. However, if the old owner comes to you and asks for their cat back, you really have no choice but to give it back unless you can prove that they pose a harm their cat. Again, in that case it's okay to not consent to the old owner being around the cat.