To eliminate even overpowering odour
Make a really strong batch of swill (Red Rose Tea). I mean REALLY strong, as in stand-your-spoon-up-in-it-strong. Let it cool and then soak your feet in it for half an hour. You will need to do this perhaps once a day for three days, maybe more, all depending on just how pungent your feet are. So find a good book, set a timer, and resist the temptation to cut the soaking time short. The tannic acid in the tea kills the bacteria that produces the smell.
To remove the smell from boots, shoes or sandals, make a smaller quantity of the vile brew, dip a cloth in it and coat the sole with it several times, allowing it to dry between applications. Repeat as needed.
If you have white soles in your sandals and are worried that the tea will stain them...well, yes, of course the tea will stain them. But unless you walk on your hands, why would you buy white-soled sandals in the first place?
For best results, treat your feet and your shoes at the same time.
I can't remember the source from which I obtained this, but it has worked for many, many people. So, many thanks to whoever figured it out.