Pax who won't strap in? Bane of my life!
If they're just being obstinate, point out all the pax sitting in front of them, and that they will collide with all of them should the proverbial hit the fan. Say it loud enough for the pax in front to hear, and peer pressure alone will make them buckle up.
If they are sitting at a bulkhead, smack that with a fist, and ask them to imagine their head doing the same at 100kts.
If that doesn't work, you can always remind them that if a seatbelt isn't an appropriate restraint for them, we always have handcuffs which do a wonderful job instead.
(hardly ever used that one)
My friend who used to be a teacher treats the pax like little children in these cases, complete with references to temper tantrums, and keeping them behind after school/landing. Its hysterical to watch.
The most common is parents who don't want to strap in their children. (
) but with a little emotional blackmail its easy enough.
"If you loved your child you'd want to keep them safe"