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-