It's not about being a party pooper. It's about setting boundaries and making sure people are aware what they are.
Here are my suggestions:
Consider what you will find acceptable/unacceptable
You need to be realistic about what you will and won't tolerate - and make sure this is communicated. This foresight will hopefully ensure you're not sat there on the night feeling disappointed by how your employees are behaving.
Will you be leading the drinking party? If yes, you are pretty much giving everyone a green light to behave in the same manner.
Some employers won't be one for partying and that is totally fine. But how will you manage if you're watching others drinking? Do you want to duck out of an event if you know it's going to be fairly raucous or do you not encourage events like this at all?
Is your party on a work night and if so, what is the expectation for arriving the next day. Late and hung over? I'm guessing not, but it's your call - just make sure that everyone knows.
Remember as well - even though an event such as a Christmas party is usually out of working hours- as the employer you're still responsible. You have a duty of care towards your employees and may be held vicariously liable if someone behaves inappropriately.
Make it clear
Whatever you decide, don't keep this information to yourself what is the point of that? As long as people know where they stand - they can abide by the rules you set. If they don't then your grounds for pulling anyone up on their behaviour are less certain.
So, send a note to all reminding them of policies which are in place - particularly policies that detail conduct, representing the Company and drug and alcohol.
Spell out what behaviour will be deemed unacceptable; being abusive to others, coming to work under the influence the next day and any specifics you wish to stipulate.
Explain what could happen if they fail to abide by the rules in place - and make it clear that disciplinary action could be a consequence.
Take action if you're not happy
If you feel that you've made your position clear and someone has mid-judged how they should behave, then you have to act. Don't wait until the New Year to express your disappointment - tell them straight away. People deserve to know as soon as possible if their standards have slipped.
If you don't act, think about the message that you're delivering: effectively that you have standards but if you break them, you won't do anything about it. Without doing this, then each event could see worse behaviour than the last!
I give these tips with the best possible intentions your employees may want to have a huge blow out Christmas do, but I can guarantee they wouldn't want that at the expense of their job.
I am more than happy to help with any of the above. If you need practical help clarifying your position to employees, give me a call. If you find yourself in the unhappy position of dealing with point 3 I can most definitely help with that.