Shoot them.
Halacha lemaaseh, jewishness, not much you can do. The real shmoozers will resent your interference & continue talking. classic aveirah goreres aveirah leading to possible lashon hara, rechilus, leitzanus, onaas devarim, the list goes on. Maintaining decorum in a shul begins with general punctuality, cleanliness, etc and begins with the Rav.