When would a spinal block not be acceptable for a csection? what back problems would make a spinal block not the best choice?

Few: There are actually very few contraindications for using them in a csection. If it is emergent there may not be time, if there is any active infectious process, if the person is on blood thinner, or if the patients vitals are unstable are the main contraindications. The frequency of their use really involves more patient, anesthesia, and surgeon preference.