You can fly fully manual, but you can't disable auto take off or landing buttons. The auto hover is controlled by the Tello. You can try placing some electric tape on the little camera that is between the 2 height sensors.
I have not tried this, but since the little camera is what makes the Tello stay in place, by blocking it, should start drifting if it can't see. Kinda like flying in the dark. If you mean you don't wish it to retain altitude, then block the 2 big lamp looking sensors at the bottom. Those measure height.
Covering the sensors worked as you predicted. Allows just about fully manual mode and take can be done manually using the motor startup and landing does work as well with stick down you just have to take into account ground effect when landing but it is still fine for what I need it for
Could you land the TELLO reliably? I only managed to land it once with all three sensors covered.
Usually it just touched the ground and then the props automatically spin up and the TELLO takes off again.