How do I stop them from breeding?

So I have 2 Zebra finches. They laid 4 eggs in March and only 2 hatched. I have the babies who are about 6 weeks old in a different cage because the parents laid a new clutch and the father was being aggressive. I did not intend to breed them right away again as I heard that if they still are feeding their babies they wouldn't lay again. Now I have 7 eggs in the nest. Just today I saw them mating again! I do not know what to do, I do not want to separate them because I do not want the mom to have to take care of them all by herself, and I will NOT kill the eggs. Some 5 of them are already darker so that means there is an embryo inside. I just want to know why they are breeding so much, and what I can do to discourage it.