Mine is not so much a park issue as a yard issue. I live in courtyard-style apts. too (somebody else mentioned it) and there's quite a few less-than-well supervised little kids around here. (Think preschool age.)
I've started bringing out quite a few of my kids' toys and parking on a chair on my patio. We have a playground, but it is small, there is no bench, and when school gets out, it's overrun.
The other day, my front yard looked like a daycare play-yard--and I was the ONLY adult with my 3. There's a little girl who's got to be 4 at the oldest who was playing with us, she's come around looking for my son, and I have NO idea who her parent is.
Anyway, in my head I've established ground rules that I'll send home anybody who is causing trouble in my yard. So far, I have not actually had to send anyone home. (I'm willing to ask a kid to share, but if you hurt someone or generally make the play not fun for everyone else, I'm going to tell you to go home. No, this area is not fenced or anything, but my children are playing in the area immediately outside our door...)
I actually rather enjoy this and it works out pretty well as a 'break' for me even.....but so far I've not had any annoying conversationalists or anyone who would cause me to "enforce" the rules. (by the way they apply to my own kids too, they are not allowed to bring out anything they don't want to share, and if one of them were generally making the time miserable, I would remove them. Not fair to the sibling if I clean everything up.)