Can't we just CLOSE ALL public parks at a certain time each night and allow night activities only with a permit?  That way, at least police have ammunition to clear out the park after closing hours.  It seems to me that if you make a park accessible 24-7, people will take advantage of that.  Yes, that may infringe upon citizens just wanting a night stroll in the park, but with all the homeless there, you probably wouldn't want to do that, so either way.