By allowing her to continue the drinking of the milk from the bottle, you have just extended the amount of time and effort it will take you to wean her (if she will only take milk from a bottle, then she is not weaned). It will be much much more of a long, drawn out power struggle now because you have put all the control in her hands, and she knows that all she has to do is refuse to drink milk, and you will give her back the bottle.
Most children at this age are very picky eaters/drinkers - her not drinking milk for several weeks while you take that bottle away will not hurt her. I don't know where you got your information, but a toddler should only be drinking 2 servings of milk a day ( that is 12-16oz) - this information is available through the WIC website as well as the american pediatric association website. Suppliment her calcium intake with other dairy, and if you really are concerned with her calcium, you can give her some over the counter Flinestones vitamins (or something like that). She should be getting enough vitamins and minerals from a healthy, balanced diet to get her through until you get her weaned. You have to take that bottle away from her cold turkey - THROW THEM AWAY. COLD TURKEY. Out of sight, out of mind. She is 2 1/2 and knows her own power ---- she also understands that the power is in her hands, and that you will most likely give in and let her have that bottle back as you did at 18 months. Be the parent, toss the bottles, and tell her that that is that. There doesn't need to be any discussion about: tell her that only babies drink milk from bottles, and that she is no longer a baby.
Take her out and let her pick out some really fun cups (she actually should be drinking out of a regular, though small toddler sized cups without a top when sitting down for meals. Sippy cups should be reserved for travel situations only) and plate/utensil deals. Might I suggest the Disney Store or Sanrio store - they have the coolest, most adorable variety of "kid" eating and drinking utensils that she can choose through herself.
She only pulls this milk power struggle because you have allowed her to do so. If you do not wean her now, which is already long overdue, you will be stuck with one of those 5 year olds that insists on drinking milk from a bottle--- which is HIGHLY inappropriate for a number of reasons that we all already know. Here is a section lifted from CA's WIC website:
Why should toddlers drink milk from a cup instead of a bottle?
At the age of one most children should be drinking liquids from a cup and no longer using a bottle.
Milk in the bottle can cause problems because:
Your child's teeth can become damaged ("baby bottle tooth decay") by the constant wash of milk, which has natural sugars in it, against his teeth. This can cause problems with his adult teeth.
Your toddler needs a wide variety of foods and that means milk is just a small part of his diet. Children are more likely to drink too much milk if it is put in a bottle.
Your child may gain too much weight.
Your child may suffer from ear infections (especially if given at night or when napping).
The number of servings of dairy foods you should eat each day depends on your age:
Children 1-5 years old need 2 servings (2 cups daily). Example for one day:
1/2 cup milk with cereal at breakfast, and
3/4 oz. cheese in half a grilled cheese sandwich at lunch, and
1/2 cup yogurt with fruit as a mid-day snack, and
1/2 cup milk in a cup at dinner.
Now go and take that bottle away, for gods sakes! Do it soon before it becomes even more of an issue in your household!