My central A/C unit freezes when my thermostat is in the off position, the outside unit is humming and ice is building up on pipes and condenser. I pull the main power breaker, that stops the freezing but not the humming,I have to pull both breakers to shut the whole unit down

Did the outdoor unit continue to run when you switched the thermostat to "off"? If so, the contactor may be stuck closed. The humming from the outdoor unit may just be the contactor, or reversing valve if you have a heat pump. Try this: reconnect power to outdoor unit only. If it starts/runs, check the contactor for fusing of the switch contacts.