I have a Honeywell XE99 Series Model T6575C (http://www.thermostatshop.com/manuals/T6575C.pdf). The fan in my condo is constantly on even if I turn the thermostat to the "off" position. The only way I can get the fan to turn off is to switch the circuit breaker off at the main fuse box.

What type of system is this (gas, electric, heat pump?). Can you provide the brand & model # of the indoor unit? Does the system function in the heating mode? The problem may have nothing to do with the thermostat.