That's the great thing about blackberry bushes. Within a couple of years of planting them, they're so high and thick, no one would be able to lie on them while their buddies ran across. The only way to circumvent them is to chop through, and that's a painstaking process, literally and figuratively. You'd have plenty of advance warning, and chances are, they'd never make it out of the bush alive.
Scouts Out! Cavalry Ho!
At our old house we had Bougainvillea which were pretty but not edible. Thorns were up to two inches long.
Perhaps the Bougainvillea on the outside row and blackberry on the inside so you can get the berries?