Spin has it right, the manual spells it out - the a/c cuts in to dehumidify the air whenever ANY setting other than "dash" is selected, and it doesn't light the indicator!
The straight arrow dash setting is the only sure way to keep it off.
I'm in the far north, I pull the fuse marked "a/c clutch", 10 amp. That way the computer thinks it is still controlling the a/c.
S