There are two options available for the disposal of liquid epoxy waste.
1. When thoroughly mixed at the correct ratio of resin (Part A) to hardener (Part B), cured epoxy is theoretically inert and can be disposed of in the garbage. The best way to dispose of unused material is to mix the two components together, let it cure, then dispose of in the trash.
2. The individual liquid components are considered chemical waste and should be disposed of properly with a chemical waste disposal company. Many local dumps and recycling centers will have waste disposal for paints, oils, solvents, ect. Epoxy can in many cases be brought to these same disposal facilities.
Note that when mixing up a large amount of epoxy a runaway exothermic reaction could take place. To prevent this spread the mass out in a thin film. A plastic lined tray works great for potting off excess material.