Under 40 pounds use a net and whack em in the narrow part of the tail. Any spinal injury will calm them down immensely.
The harpoon / flying gaff is my favorite for the occasional bigger butt. I try to hit near the spine about one third length back from the head. Poon tip tied to 20 foot of rope a good sized float attached and more line to tie off to cleat.
I`ve heard of a spot near the head that will cold cock them but haven`t found it yet.