Hi,
I know you can kill the trees and shrubs, but make sure you are doing the right thing!
Grass does not stop erosion!
Iceplant can add to fires and erosion over time.
Cutting simple garden paths can slow erosion.

Some trees and shrubs will hold the soil a lot better than grass- and you don't have to babysit them by mowing, watering and feeding constantly. Not to mention that mowing a hill is difficult work.
You can kill shrubs and trees with a product called: @Ortho 32 oz. Max Poison Ivy and Tough Brush Killer Concentrate-0473010 at The Home Depot It is used on the bark only it does not work on the ground

The planting needs to be a mix of ground-covers, shrubs, trees, and perennials with the areas between plants(if there are any openings) covered with mulch and/or boulders. A varied planting is FAR more effective than grass on a slope!
Do not change the grade around existing trees-you will most likely kill them.

Hummingbird sage and Coyote bush[ Baccharis]are great soil holders, and very low to the ground.
These plants hold soil from erosion:

Try this site for some great suggestions!@http://www.laspilitas.com/garden/howto/slope.html