Depends. Stent, surgery, or close observation.
Conservative. Spontaneous ones can be treated most of the time with anticoagulation like Coumadin (warfarin) for few months till dissection heals. Some patients who fails this with recurrent TIA or aneurysmal degeneration will need surgery or stenting depending on the location and extend of the dissection.
Stent. Most often these days placing a stent in the carotid artery will do the job. Similar to stenting coronary arteries.