You could try breaking the neck? I feel like that's one of those things that could be inhumane if you don't know what you're doing, but as it is I feel like it'd probably be more humane than the current method you're using. This is just speculation as I've never pre-killed or disabled my live feeders. I usually just wait with a pair of tongs to put into the prey's mouth so it can't bite the snake as it's being constricted.
I'm sure someone else with more experience can answer this better.