Occupying reinforcement hexes?

[fluidwill]

Most Land units will always appear at the location given in the
Formation Report, if that location is available. A location is only
considered to be available if it is either friendly-controlled or
located on a map edge. If for some reason the location is not
available, units scheduled to appear on a map edge will instead
appear at some other available location along the same map
edge, within two hexes. Units scheduled to appear away from a
map edge will only enter when the location becomes friendlycontrolled.

Fromthe manual,pg 28...

From reading that I'd say control of all 5 hexes (designated plus 2 each side?) is enough to deny arrival, I've never tested it though and am happy to be corrected.