If you would REALLY like to keep everything out, you could purchase one
of those electric horse/cow fence kits. Wire it so that the wires are
about 4", 8", and maybe 18" above the ground and plug it in. They don't
generate enough voltage to cause any damage, but they hurt like hell.
We have them here for our horses, and nothing goes in or out of that
pasture beside through the gate. The dogs stay WAY clear of that area.
Just a perverted/strange suggestion which does have some merit.
JMS