This is another nice and easy one, though it may take a while depending on the size/speed of the tape:
sudo mt -f [/path/to/tape] erase
sudo mt -f /dev/st0 erase
That’s /dev/st(zero) not /dev/st(letter o), in case it’s not clear from the font. This will likely take some time as the tape is erased from end-to-end – once it is done you will be returned to a prompt.