A lot of amatuer radio operators and RF engineers are telling me
that such a
device could wreak havoc world-wide

It won't "wreak havoc" world-wide, but it is very likely that it will
be heard on occasions long distances from your location - and it is
illegal. Even if your signal is only heard by your neighbor,
intestinally jamming him is not legal - even if what he is doing is
also not legal. On the technical side, it is just not possible to
generate a signal that will be strong enough to block his on your
property while ensuring that your signal will not leave your property.
It is not technically possible to control signal strengths (especially
given that your talkies will be moving around the property) to the
point that it will block the reception of his signal while letting
your signals through without interference when using the same
frequency.