Ok, I am sorry but I don't understand why you are having these issues, My good friend used the avent bottles and did not have that problem. Maybe it is because she didn't use the bottles to mix the formula in. She bought a container to mix it in because the avent bottles have the rubber peices at the bottom that the powder can get under which would be a BIG reason for the bottles to leak.
I beleive the instructions tell you not to mix the formula in the bottles.
Oh and I am with Stealth, I wouldnt throw the bottles away I would try to get my money back.