The above answers are both very helpful. Perhaps additional specifics may be, as well. I always encourage a multi-step process in these matters.
First, make a formal "sit-down" appointment with the physician to voice your concerns. Bring a support person with you, so that you are not alone in expressing your views and it is clear that someone else is aware of the issues. The appointment is important to ensure that your concerns are not relegated to a "hallway chat" where the discuss may be overly rushed or distracted by other obligations or interruptions. Such a meeting may also reveal underlying disagreements about treatment, etc, that can then be promptly addressed and resolved.
Second, if the formal discussion does not succeed, request an "ethics consultation" on the matter. Nearly all hospitals today have ethics committees, with members on-call to provide consultation and support to patients, families, and physicians. Sometimes a brief intervention by an ethics committee member is all that is necessary.
Third, if a "consultation" does not resolve the concern, request a full Ethics Committee review. This requires a full quorum of committee members to address the concern. Most committees have a diverse membership, and special training on advance directive matters. Other options include contacting the hospital's "Patient Assistance" department, and/or contacting a local health care "ombudsman" to intervene.
Finally (and reserved to last) are steps such as lodging a formal complaint with hospital administration, contacting the local medical board, or seeking formal legal advice to press the matter.
Advance directives are legal documents, and they should always be honored. Those named by the patient to ensure their wishes are honored have an obligation to honorably discharge the duty.
One final note. Having an advance directive that is very clearly written, and that very plainly spells out the role and authorities of the person named can be particularly helpful. An example is the Lifecare Advance Directive (see: www.LifecareDirectives.com). Where more vague standard directives are used, additional supporting material such as that found via the American Bar Association "toolkit" can be helpful, as well (see: http://www.abanet.org/aging/toolkit/home.html). Exploring all document options in advance can help avoid similar problems in the future.
-- JT McKay, PhD