Maybe try decapitating them, since it is possible to cut them.

They take a minute or two to heal, it takes a second for a guillotine to chop em.

A guillotine is surprisingly easy to make, all you need is a table, wood, rope and some metal sheets from the local hardware store.

Ofc you probably won't have time to make that contraption, so perhaps just aim a blade at the neck.

The kid got knocked out, brain damage is definitely possible it seems like. Blunt force to knock em out, then executing them might be the only way one might immediately try.

There's still a chance fire will work. They can be physically damaged, and fire might damage them faster than they can heal. The problem with the kid is fire also burned those zip-ties, so he could just waltz right out after maybe a few seconds in the flame.

Stab rebar thru them and bend it in a cane so they can't escape, then see if they do burn out to the bone eventually.

I don't know why I'm thinking about all this-

One time I saw that mountain with pink clouds, hanging low enough that the mountain casted a shadow upon the clouds. It was an incredible sight, I've never seen a mountain cast a shadow on clouds before. The calming color of the pink and orange clouds, swirling in their playful fluffs- as if they swirled like milk pouring through coffee. The sun hung low, perhaps that's what made the shadow so long, as the mountain's tip disappeared behind the clouds. I'll never forget the view, as I drove through town, as I drove through fields, but perhaps what sight I favored most, was the face of awe on my friend's faces. Only by chance were they also in the car, and lucky did they get as they wouldn't have seen it otherwise. There will never be sights as enjoyable as could be, without friends to share it with. I wish I could hear them asking for a share of snacks one more time as our eyes stared onwards, perhaps... I shall talk to them soon, once I'm well rested.