Somewhere in the basement, I have tape eraser that was recommended by one of my brothers (who ran KHOU's Sports department for 30 years and it was a piece of gear they used all the time for professional video tapes). I haven't used it much, but I have used it for 9-tracks and seems to have worked. FWIW: I got it here at a (in)famous local electronics place near MIT (Eli Hefferon and Sons - 'A Nerdi Knight Holy Place') I must have gotten it in the early/mid 1980s for about $75, I remember it was much more expensive than the Radio Shack thing - but my bro said it really worked. At the time I was doing some consulting for a NYC law firm and I had to document to the courts that all the data I had been delivered was properly destroyed. They were satisfied and never heard anything more about it.
It seems to have erased everything have had near it when it on. In fact, I was always a little worried that it will erase unintended stuff too when I use it. It hums and vibrates on the table and you are supposed to move the tape horizontal to it and make circular motions over it. I had taken my watch and wedding ring off the first time I ever tried it and it moved my watch from a table about 3 feet away. I was pretty impressed.