I've found the total vegetation killers, eg Triox, to be very
effective. Properly applied, they will last for months, up to a whole
season, but nothing will last for years. If you applied the product
and then put down other material on top of it, ie cloth and rock,
you;re doing it in the wrong order. The product should be applied on
top, as a final step. When you put cloth and rock on top, after
applying the product, there is going to be some amount of dust and
other material which is now above the killer, leaving a place for weeds
to get established.