What is the best/safest way to dispose of unused/expired casting material?

The best/safest and only way .... is to mix the components A & B together as if you were going to use it. Put it in an oversized container and do it outside if it is very old, in case the moisture contamination is so great it could create a rising foam and go up, over, and out of your container. Trust this... from someone who has experienced that, (smile) it could be a very bad thing on a kitchen table or inside floor!! You can then dispose of the cured piece if you take it out of your mixing vessel, or if it bonds dispose of the cured material with the vessel. An old ice cream pail, coffee can, paper cup (if very small amount) whatever you have on hand that you don't mind disposing of will work, no styrofoam of course as it will melt through that!