Girdling the tree (Seven’s suggestion) would be a very fast and simple way to kill the tree without poison if you weren’t going to cut it down anytime soon. If the logging co.s were going to try something like this, that is how they would do it.
OTOH, regardless of how you kill it, dying trees have large, dead, lower branches that easily fall off when you do cut them down. Such branches are called “widow makers” by loggers. Now do you see the down side of killing the tree but leaving it up?
Just send some fire ants up the tree 
Yes, I do see the downside. However, if you kill the tree, even if you just leave the nut up there, would it not be very demoralizing? Would it not make sitting up there, in a dead tree, seem really pointless and silly?
Does anyone know where I can find a list of current tree-sitters?
It is also illegal.
You also get in loads of trouble if the person falls down from the tree due to your actions.
Ok, just so everyone knows, cause it doesn’t seem like so far, I do not heartily endorse this product or event (hippie tranking).
Except in extreme circumstances…like if it’s a rabid hippie or something.