I'm glad you said that you don't believe in this type of punishment - most of all, because it isn't effective. Especially in children with Aspberger's/Autism. As you know, they have difficulty making human connection and are generally socially awkward. Additionally, they don't have a great understanding of the social contract between people that it isn't okay to be violent - they don't have a good understanding or grasp of the consequences. Empathy is difficult for children with Aspberger's. Therefore, if you react to him with violence, it is demonstrating exactly what you don't want him to do to others, and solving a situation in a socially unacceptable way.
Holding firmly (think gentle bear hug) and calmly should de-escalate the situation acceptably. You're not harming him, but you are stopping any further physical damage. Then communicating quietly and calmly that his actions were not acceptable should follow. What are your thoughts?
No problem! I know that raising a child with Autism or Aspberger's can be very difficult - especially as they get older and physically stronger. I wish you the best of luck,
PS - if hugging him with his face toward you is too activating (it can be for some Aspberger's kids depending on how much eye contact they can make), you may need to hug him with his face away from you. Just a tip. :)