Before we talk soundproofing--is this land public, as in owned by the city, or is it private, as in owned by your apartment complex? If the latter, complain to the management.

And continue to call the police. Try calling their non-emergency number and asking them what to do about the situation. They might have some advice for you, or another place to call that can actually get something done.

If you see any evidence of drug dealing or underage drinking, I'd mention that when I called the police, not the noise.

Also, contact your local city councilperson, or alderman, or whatever your town has, and ask for their help in addressing the issue. All the parks in my city close at sundown, so anyone found in them after dark can get ticketed by the police. Check around and find out what the regulations for noise are, and for parks are, in your town.

As for soundproofing--if you own your own place, you can replace the windows with special double-paned windows that are designed to help reduce outside noise.

You can put heavy drapes over the windows. They can be open in the day and closed at night. Bookcases full of books on the outer walls of the rooms closest to the "park" can also help stop noise. And there are white noise machines that you can buy that might help you get to sleep when the parties start up.