1. cinnamon oil - this is an excellent option for you, as it kills adult spider mites on contact, and is also effective at controlling, but not eliminating PM infestations. mix 1/2 tsp. cinnamon oil with a few drops of dish soap and mix until milky in appearance, then add 1 quart of room temperature water. test on a small area of the plant first, as this has the potential to burn the plant.
2. hydrogen peroxide - again, this likely will not take out the entire infestation, but it's safe for use in the garden and will effectively remove around 70% of the infestation
3. milk - this is the choice of many organic gardeners, and most rose growers use it as well due to it's astonishing effectiveness. most people mix 1 part milk to 9 parts water, then spray the entire plant. beware that if using strong milk concentrations there will be a different mold that forms, but is harmless to the plant, just no good for smokers. if this occurs lower your milk concentration.