I specifically told the assisted living not to give my mother antidepressants and they did it behind my back! Now what do I do?

The medications would have had to have been prescribed by a doctor or other qualified person, such as a physician's assistant. Do you have authority to talk with her doctor (did she signed HIPAA papers giving you access to her health information?) If so, you can talk directly to the prescribing person about this. If not, you will have a harder time making changes.
Either way, there must be some reason why the medical people think she needs this medication.
Please attempt to talk with the doctor (or other prescribing medical person), as the nurses or CNAs in the center are only carrying out orders. Maybe you'll see evidence she needs an anti-depressant. They do help many people. If they can't show you why the drug has been prescribed (assuming you can see the records), then you can discuss what she really does need, if anything. Good luck,
Carol