I have several strategically placed Berberis plants under most of my windows and I've grown up a hedge of them onto the perimeter wall at the front of my house.
They look really good, but having had to pot hundreds of these damn things years ago when working in a nursery I have grown to respect them. Get the right variety and not only are the leaves sharp enough to cut like razor wire, there are some nice 3/4" spikes hidden in there too that really are not noticed until it's too late.
As far as putting signs up - go for it. They won't stop a pro but as a deterrent for the price they may well help.
Nothing beats a loaded shotgun in the paws of a pit bull with a bad attitude and a headache though.