Aprilaire 600M Wiring to Honeywell TH8321 Stat
I skimmed through the thermostat manual in the link but it didn't seem to address your model very well. For example, I didn't see anything about the capabilities of the U1 terminals.
The signal from the thermostat to the humidifier to quit humidifying is simple: the thermostat's output controls the water valve. When the thermostat turns off its signal the water valve closes and the humidifier dries up.
A simple solution is to get the Aprilaire humidistat and let it control the humidity rather than having the thermostat do the job.
It's a little nicer to have the thermostat control the humidifier valve, but to get there it looks as if you'll have to find some other way of controlling the damper. An upgrade of the thermostat to one that supports two accessories could work. There might be another way, though. It depends on when the damper should be operated. The furnace control might already have support for operating an accessory, for example. Or, if the damper should operate only when the thermostat calls for heat, you could add a relay to make it so.