Scrape them off. The willows will thrive as long as the tips can see light. Spraying them with glyphosphate products now probably have little effect as the plants need to be actively growing to kill them. As I found out this summer, only one year of growth can lead to a quite extensive root system that will sprout more limbs if exposed.
Definitely don't use diesel, sort of a bio-hazzard. Very long-lasting in sub soils.
