I take it that your iron balls are hollow and made of rust resistant steel. It would be a fiendishly expensive project, but I doubt it would have the result you suggest. The iron balls would have to be linked together by chain or rope to prevent them dispersing in ocean currents and ending up on beaches around the world, like the cargo of plastic ducks which fell overboard from a ship about ten years ago.
Such a contrivance would represent a hazard to shipping, so no government would be willing to spend the huge amount of money it would cost. The whole thing would probably end up on the beach of some Pacific island and cost a fortune to remove.
The problem of plastic in the ocean is certainly a serious one, and something needs to be done about it, but this is not the solution. Perhaps an internationally financed ship dedicated to seeking out rafts of plastic at sea and disposing of them might help, and would certainly be much cheaper and easier to control than your million tons of iron balls.
A world-wide educational campaign aimed at teaching people the damage caused by dumping their rubbish on beaches, in rivers, and from ships might have some effect in reducing the amount of rubbish which ends up in the sea. I always use an ordinary plastic shopping bag a couple of dozen times before it starts to fall to pieces, and then I put it in a bin bag to go to landfill. If everyone did the same it would be a major step in the right direction.