Cutting ambient is pretty much your only option with your current setup. Other solution would be HSS which isn't very popular among sports shooters, but i'm not really familiar with it. Or get/borrow a camera with an electronic shutter (d40, d70, some others,) and have a higher sync.
And yea you just have to stop down to cut ambient, which is going to strain your flash power/capability, but it's all you got. If you don't have enough flash power, either wait till the weather's darker, or borrow/mooch some monolights and be able to overpower the sun.