In my humble opinion any work related event, especially if officially organised, is subject to the same rules of conduct that apply in the workplace. I strongly suggest that you should report this to your manager or HR department as an official complaint and let them deal with it in the appropriate manner.
If this is the first recorded transgression, the offender would normally be subject to a verbal warning. If this is a second or subsequent transgression then it should be dealt with as per you employers stated term and conditions of employment.
At some point someone has to draw a line in the sand and inform the miscreant of appropriate levels of behaviour at work, for his own sake.

Ian