No-one has mentioned gooseberries which have vicious thorns.

Actually, for maximum deterrence the plants not only need thorns
but should be clearly seen to have them.

My favourites are Rubus Cockburnianus (ghost bramble)
http://davesgarden.com/pf/showimage/4179.html/
and Rosa Omiensis Pteracantha (r.sericea pteracantha)
http://www.rosesloubert.com/collecti...20omeiensis%20
pteracantha.jpg
which has very scary looking blood red thorns.