to first ensure that you have a clear set of behaviour guidelines (including but not limited to policy on prevention of bullying, harassing and discriminating behaviour and a code of conduct or “house rules” or “values”)
In those policies you need to clearly state that inappropriate behaviour will not be tolerated and that it may result in disciplinary action.
Secondly if you hold a party then make sure that alcohol is served responsibly and by accredited staff (ie bar staff who hold an RSA certificate)
The days of the open bar and a free for all are long gone in the world of the responsible employer.
Third, members of the management and leadership team – that includes you – need to be present and role modelling appropriate behaviour. You can have a drink or two but should remain well and truly in charge of your judgement..
Finally, encourage staff to enjoy the camaraderie rather than merely free booze. Perhaps a sit down dinner with drinks is a better option that a night out at a pub enjoying a band.
It’s important to gently remind staff that the Christmas party is a celebration yet it is still a work function.