Well, taxation is theft, so that is the easy answer to that. :-)

But to dig a little deeper... a UBI is an interesting concept, and especially so when one considers a future where most, or even all, human labor becomes unneeded. In a world of 100% technological unemployment, it does seem that we're going to require a different system than what we have now. But I don't think just saying "have an UBI" is much of an answer, because - IMO - the entire fundamental basis of the economy will be so radically different that we can't even imagine how the UBI will be funded and how the economy will work.

I keep running through scenarios, trying to come up with a scenario that makes sense, even hand-waving around the "initiation of force" issue temporarily, and it always seems to come out looking like a Ponzi scheme in the end.

Luckily, I think we're still quite a ways away from the total obviation of need for human labor, so we have some time to figure it out. Well, hopefully anyway.