The dead flowers have seeds. So get the seeds from them. The residue plus the leaves and stems could be soaked in hot water for a spray. If you need it in the spring, store them dried in a plastic bag till spring. And spread those seeds wherever you want marigolds. IMHO. LOL
"Nematicidal compounds apparently permeate from marigolds’ root tissues into nematodes attached to the root, but they are also believed to kill nematodes found in the rhizosphere, the soil near marigold roots. Thus, marigold is believed to be most effective in suppressing plant-parasitic nematodes when actively growing, but it is not as effective when incorporated as crop residues or root extracts."
So maybe what John said is right on the money- harvest the seeds and plant those marigolds elsewhere!
Common Weeds And Wild Edibles Of The World (HD video)