Not sure if you do this stuff already, but a couple of quick suggestions:
1) Have them in enclosed spaces
2) Require advance registration
3) Work with space owners / managers to enforce a policy excluding
non-registrants
4) Establish a policy that prohibits harassment or heckling, and
empowers the chapter to bar those who violate the policy
5) Strongly and repeatedly encourage attendees to immediately report
inappropriate behavior
6) Escort those behaving inappropriately to the door, revoke their
registration, and bar them from future events