I actually don't see it as too much of a stretch to charge someone who is knowingly shooting hot with battery/assault. The whole will-fully causing harm and whatnot. It is one thing to be hit once or twice by a standard marker as is stated in the agreed upon rules, but it is another to be actively attempting to cause pain and suffering.
Talk to the right lawyer and I can easily see the field owners facing criminal negligence charges, and losing.
Personally I would start at suitable local government office and talk with someone there. If idiots want to get together privately and make each other bleed, then fine. But if they're doing this as a public field, that is a serious safety hazard, and needs to be shut down.