I know this is an old topic, but hopefully this will help somebody.
I've found a way to hide the channels but keep the Math function. This was driving me nuts!
It's pretty simple:
Press {DISPLAY} {INTENSITY} then lower the value to 0%.
You will still see a slight ghost of the channels, but it you really want to, you can also select {TYPE} DOTS and {BRIGHTNESS} to less than 30% and the ghosts of the two channels go too.
Thankfully the {DISPLAY} menu only affects the displayed channels and not the MATH function display. So luckily this bug cancels out the other bug.
Cheers,
Trys
PS In the image below, I'm multiplying CH1 and CH2 which have 12MHz and 7MHz to show the behaviour of a Diode Ring Mixer. We can see the MATH function shows the product and difference of the two signals, and the CH1 and CH2 are hidden.