Hi B.,
it's a tough spot to be in, but I think ultimately, just refuse to give it to her at certain times. She'll probably refuse to drink anything else, but eventually she'll get thirsty and drink something. She won't be dehydrating if she doesn't get it. My daughter struggled with GI issues and got hooked first on PediaSure, then on Carnation instant breakfast and it was a tough road to get her off those. Still, she wants chocolate milk and when I tell her no and offer something else, she stands her ground. Fine by me. But inevitably, she comes back and asks for water or juice.
Set in your mind how many cups is enough (3 maybe?) and stick to that. Another thing I try is to be somewhat random so as to break her habit. So sometimes, I won't let her have milk in the morning and sometimes I do, and she never really knows and so I think it's a little easier to let go of the disappointment since it hasn't been built up by the routine. Granted, she's a little older, but I still like that idea.
Remind yourself that you know what's best for her, so even if she's upset, you still need to do your job.
Good luck!
~Liza
30yo, full-time working, single mom to 4.5 year old girl.