1) My first group outing (1984), an October crossing of Barnegat Bay, was hosted by East Coast legend Chuck Sutherland. The day was predicted to begin in fog and calm then morph into sunshine and a brisk SE seabreeze. A paddler arrived with a newly-purchased used Nordkapp, from which he had removed the seat, as he had found the boat too tender with the seat in. Also, he had no sprayskirt. Chuck, radiating quiet authority, took this paddler aside, out of earshot, and persuaded him that he was not appropriately experienced or equipped for the trip. The kayaker took it well, and watched us push off.
After crossing the bay and climbing Barnegat Lighthouse, we returned in a vigorous beam sea, as forecast. We often took large breaking waves over our sprayskirted cockpits that would have quickly filled an open cockpit and sent such a paddler over very quickly.
2) One of the most versatile and reliable kayakers in our local club remembers vividly, as a novice kayaker, being left on the beach by the leader of another club very early in his paddling career: Too inexperienced, and not dressed for immersion. The episode left him steaming for a long time, but, with the passage of time and more exposure to the sea kayaking literature, he determined to become someone who would be at home on trips, and maybe lead trips himself. He is now one of the most capable, experienced, and thoughtful paddlers in our club (and is way better than me).
3) I've never had to turn anyone away from any trip I've led, but that's mostly because I so detail the conditions that might be encountered, that few actually show up at the launch site. So the "intervention" I'm proudest of is a long club message board campaign/debate wherein I successfully halted a proposed transit by club sea kayakers of the Point Pleasant canal here in Jersey. The canal is busy, very narrow, fully bulkheaded, and boasts currents of up to 9 knots--not an ideal place to capsize a sea kayak; we'd get a real black eye, should that happen. Later I led and won a fight to not oppose an ordinance banning manually-powered craft from the canal. Jet skis had already been banned, and manually-powered boats were even less suitable to transit the canal. We were then spared the opportunity to look like morons, had we opposed the ban (which was going to pass, whatever kayakers thought about it). In fact, it was the explosion of kayaks on local waters that had triggered the formal ban; it never had been thought necessary previously.