Linda wrote:

What's the quickest way to humanely do this?

cruel heartless ******* warning lol, wait till you have to do a cat.
Trying to bust their head with a sledge just busts your path up.

There isn't really a "humane" way.
You could either put it in a bog some where safe from the cat untl it
dies, or drown it.

At one stage you could get real chloroform from the chemists (regular
use for rabbits with mixo). now all you can get is chlorowater which
does nothing.

You will probably never have to do it again, but if you could try the
rabbit neck stretch method. Hold body in one hand, pull head down with
other whilst rotating head backwards until it cracks = one dead bunny.