i'm wondering if there is a low-lying shrub/ground cover that has thorns. i specifically want it to deter transients from the front of my business, but we have a large mural that i don't want cover either. hmmmm??? any help will do...

Barberry....
You can trim them as you need, will
spread low to the ground and take pruning nicely and has THORNS!!!!
(a pretty bonus has beautiful color is the Japanese Barberry)

There's several varieties also...

Oceanside's climate zone is 24 according to Western Garden Book. As Weezie suggested, Barberry might be a good one. Look for MYRICA california/Pacific Wax Myrtle[zones 4-9, 14-24] evergreen shrub.

picture and info at:
http://www.laspilitas.com/plants/447.htm