I turn the thermostat to OFF but my a/c outdoor unit keeps running and won't stop. HELP

It is a good possibility that the problem is in the outdoor unit. There is a item called a contactor and it takes a 24 volt signal from your thermostat to activate it but many times the contactor becomes stuck due to a shorting of the power contacts and that causes the outdoor unit to stay running even with the thrmostat turned to the off position. I suggest you kill the power to the outside unit until it is repaired. It looks like these