I have no idea how you keep a ride like this from just turning into an unregulated road race.

We have a few unorganized/leader-less rides like this in my area. The race-like atmosphere around these rides is part of the attraction. No one is in charge, people just know to show up at a certain time/place and every week it's the same route, same distance, same sprints. Some of these rides have been going on for years with no one really running the show. Lots of local racers show up and sometimes the bigger teams will try to assert some authority over the nonsense, but often things can get out of hand (think red light running, attacking through traffic, riding 4-5 wide and even into oncoming lanes, etc).

These rides (especially the weeknight rides) are notorious for being very hard and also having lots of crashes. There are splinter groups that have formed to start just before or after the main rides to try to avoid some of the chaos.