Thanks Merav. Points well taken. I've actually replayed the conversation a thousand times in my head, each time leading towards responses to the effect of those that you suggested.
For work events and other things I have some time to plan for, I do have some pre-planned responses for turning down booze. This caught me totally off guard. Ding dong goes the doorbell, open the door and there's someone with a bottle of vodka complete with a bow wrapped around it.
However, as they say, hindsight is 20/20. Or my personal favorite, "If ifs and buts were candy and nuts we'd all have a merry Christmas."