There are cameras everywhere at ballparks. I am sure that they can figure out who is throwing things onto the field. Happened in the Giants-Padres game yesterday in SF when fans were throwing things (including plastic bottles & baseballs) at Jurickson Profar of the Padres.
Make the announcement during the game and publicize it in the local media. If it happens again, we'll be able to see who is doing it, and you will be banned for life from the park - not only that, but if you do it again, alcohol sales will be banned for the entire outfield sections, as well as the kiosks near those sections and you'll have to walk to the other side of the stadium to get your $16 beer. Then outfield netting will be put up in front of the offending section - and your name will be publicized and you will be given full credit for the changes. If you think that's worth becoming famous, understand, that people will hate you and harass you constantly. Nothing sadder than watching home run balls get caught in a net instead of into the fans. I understand why they have it in Toledo (don't want a car to get hit), but at a Major League park it would be embarrassing.
Fan behavior in sports has gotten totally out of hand. It's always just a small group of people doing it, but it ruins it for everyone and drastic changes will have to be made.
Another indication that our society is falling completely apart.
I don't go to NFL games anyone because watching at home is a much better experience. Last game I went to at Ford Field several fights broke out. Morons. ADWB Angry Drunk White Boys - angry about what? If you can afford an NFL ticket, you're doing alright.
If I see someone throw a battery or bottle onto the field at a baseball game, you can bet I'm snitching on them.