Plants have been used for protection down the centuries, and one of Mrs F's comments made me think it might be an idea to print a few general guidelines to using them nowadays.
Basically, anything aromatic is generally protective. So you can use rosemary, lavender, and of course, bay. Haven't you ever wondered why people put Bay trees outside doorways? The areas of entrance and exit to home, cattle shed and stables were always considered the most vulnerable, and needed protecting. Bay had the added advantage of protecting the home from lightning, too.
If you want more than general protection because you consider you are under psychic attack (another favourite topic of mine 
) then consider a plant with thorns. Again - haven't you ever wondered why people planted roses around cottage doors? Roses are both aromatic and have thorns, therefore they could be relied upon to protect those within the house.
Here's a pen and ink sketch I did of the lovely Dog Rose, last summer:
2008-04-02 @ 13:39