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I have an old Honewell mercury thermostat with a a/c and heat back plate.
It's been there maybe 20 years. I can turn the fan off and it still runs where it's on hear or cold. The heat is hot water baseboard. Is there a reset button on these old thermostats?
The only time I've had trouble with things not shutting off was with digital types with failing batteries. They would get really screwy when the battery was low.
If the fan switch is in auto, does this still happen? In auto, I thought the device (furnace/boiler/a/c) controlled the fan. It may be wired wrong. For heat-a/c-fan control, it needs 4-wires. Essentially, hot, heat control, a/c control, fan control. The internal switches connect the hot to either heat or a/c control, depending on the switch position and the thermostat setting, and can also connect to the fan control line independently.
I have a trane model twg025a 140b1. It's about 12 years old. The thermostat wires go into the unit. I suppose when I remove the side panel I'll see what's inside. I had a similar problem with my gas boiler and the thermostat wouldn't turn on the circulator pump. Turned out to be a relay
on the side of my boiler. To bad the A/c isn't as easy to find.