If Damage is Caused. Raise Taxes.
... If Milwaukee County's parks are being damaged by Pokemon players and there exists good evidence that that is the case then raise a Pokemon Tax on Milwaukee County Residents.
I know nothing about Law or American Law but I doubt that raising Taxes to maintain Recreational Facilities violates some Amendment or another. Tax the Locals harder and see if they amend their behaviour.
Might cause a bit of grumbling by the people who do not play Pokemon... Or riots in the parks.