Allowing that purchase and then mixing of resin and hardener in the correct proportions leaves little if any unused, what is the best way to dispose of whatever is left? I imagine mixing it up and letting it cure (such that it is rendered inert) before disposing of it would be ideal. Locally (to me) there are hazerdous waste collection days once or twice a year to collect and properly dispose of paints and varnishes, pesticides, pharmaceuticals and other household chemicals that would are potentially toxic if added to the landfill. I'm moving after seventeen years so the topic has been on my mind as I rid myself of an accumulation of unused stuff.

If I had 'usable' quantities of epoxy and hardener (say, >500mL) I'd probably try an online listing in the 'Free' ads in my area. Somebody might be able to use it. I'm not very concerned about strangers coming to my door, but that can be an issue in some places.

You are lucky if you have hazardous waste collection that is free. Here, (Victoria, BC) we are made to feel like 'minor criminals' if we use paint or epoxy; it's a hassle to dispose of empty containers 'properly', let alone left-over materials. Of course that doesn't keep the 'authorities' from imposing an 'Eco-Fee' on every purchase!