How do you turn off a specific bit in a bit mask?

Found it! Use & ~ like this...

UPDATE MyTable SET
        MyBitmask = MyBitmask & ~128 -- 8th bit
    WHERE MyID = 123

The ~ operator flips all the bits (1s become 0s and 0s become 1s). Just set the value that you flip to the one you want to turn off and use & to safely turn off just the one specific bit without having to check to see if the bit is set.

You could also use: MyBitmask &= ~128