You probably can't sterilize the straps that accompany typical headlamps. That would require certification of a procedure. It would probably require autoclaving (steam pressure which you could replicate in a pressure cooker), ethylene oxide treatment, or gamma irradiation. (Don't try these at home.)
You will have to settle for sanitization or disinfection. There are basic chemicals or chemical groups used for disinfection, phenols, halogens (including hypochlorite bleach), quaternary amines (quats), peroxide and alcohols. You choose based on what you are trying to kill. Forget phenols. Anyone remember Lysol? It is not so commonly available anymore and they've changed the formulation. Just forget about it. There is a key word that you can look for, tuberculocidal. I am not sure of the current (OSHA or other) regulations, but it used to be that any cleanup of materials potentially containing blood-borne pathogens using tuberculocidal agents is OK. There are disinfectants that are not tuberculocidal , but OSHA-approved for bloodborne pathogens. Note that any disinfectant must be used on CLEAN materials. Soil of any kind can interfere with killing the pathogens.
Bleach is nasty. It is probably the most dangerous material commonly found in households. It quickly wrecks a lot of organic stuff like natural rubber and synthetics. It would probably be OK with limited cycles so OK in an emergency kit.
Alcohols have their own problems, flammable and volatile. You need 70% ethanol or isopropanol. Ethanol is less toxic to humans, but denatured ethanol maybe not since it has added stuff to make you sick. (Lots of formulations out there, hundreds if not thousands. I once had a 5 gal can with, among other things, had gasoline added as a denaturant. The revenuers controls these things.)v Isopropanol does not evaporate as fast and with any of these things you have to have contact for 10 minutes. (The EPA approves disinfectants and their procedure mixes the stuff 24 hours before testing and uses 10 min contact time.)n They don't clean in any way and are not tuberculocidal.
Then there are quaternary amines. That is what you will find in, probably, any and all cleaning agents on the grocery store shelf. Trouble is, they are probably not tuberculocidal. but there are tuberculocidal quot products and you an probably order on line. I use Decon Labs
BDD™ (Bacdown® Detergent Disinfectant)
Easy peasy, not terribly toxic or irritating, cheap, and probably won't hurt the straps. Just dunk them for a half hour agitating now and then, rinse well and dry.
Hydrogen peroxide will also work, but I'd go with a quot product.
