The idea isn't necessarily that you explicitly null-route them but
rather that you block/ignore announcements of them on the assumption
that malfeasants may be attepmting to squat on them or otherwise use
them for some form of, well, malfeasance.  As such, the filter you build
isn't just e.g. "2a10::/12" (if indeed that range was to be considered a
single bogon) but rather "2a10::/12 ge 12" which means you'd block
more-specifics within that range, too.