Once a child is adult or semi-adult size, this is not "spanking," this is hitting, fighting or a physical altercation (if they hit back). I am not opposed to spanking in some cases, with a less than five year old child. I think physical respect is the basis of all discipline,ultimately. I would certainly not allow a child to hit me; physical restraint and involving the authorities (with a teenager or large child) is the most appropriate here.
However, when a child is a teenager, hitting them, no matter how frustrated you are with them, is not appropriate. I have taught parenting classes for my state. Smacking, hitting, spanking, whacking, slapping, etc. (or throwing back the covers and wearing his butt out with a belt) is not considered legally appropriate discipline for teenagers. Alabama, this is not "spanking." This is assault.
The best thing you can do, if this is pursued, is explain that you are at the end of your rope, that you did not know what to do, and that you have tried all the things you have mentioned, such as grounding, taking away privileges, and other inhibitors and they have not worked.
If you go into this with the attitude "I have a right to smack my kid around, because he or she is my kid, and I was whomped as a kid and my daddy wouldn't have put up with this stuff for a minute..." you 're going to come out very poorly in the family court system.
Think very carefully. Did your daddy really whomp you a lot as a teenager, or were you just, by the time you were a teenager, so thoroughly a believer in your dad's authority that you didn't really try all these things because you believed he would have whomped you upside the head?
If you were hit a lot as a teenager, you probably have huge authority issues and lots of lingering bitterness and problems. But many times, when you talk to people who believe they were physically disciplined a lot, they can't really come up with even one incident (especially when they were older). They just felt like that would have happened if they had ever dared to do some of the things that kids do today.
It sounds as though your teen is truly a little out of control. Hitting them isn't the key to reestablishing control. Asking for help from the authorities you have been reported to, and appearing willing to accept that help is probably going to work best for you in a court or Family Services situation. What is "the law?" is going to be very subjective. This matter will be put into the hands of CPS or a juvenile judge, and both or either of these entities will be trying to figure out what is appropriate in this situation, and if the child needs to be removed from the family situation at present. Your appearing to be a reasonable concerned parent is all it will take to improve things here.