How do I remove sticky or uncured epoxy that never hardened?

That depends on how soft it is.

If UltraClear epoxy is hard but slightly tacky, sticky like spilled soda, it doesn't need to be removed. Pour a new, correctly measured and mixed flood coat over the entire surface and it will cure hard and clear, with no sanding needed. For a few small tacky spots, you can instead paint a small, fully mixed batch over them with an artist's brush, dipping from the center of the container rather than scraping the sides. (This fix is for our epoxy over our epoxy; tacky epoxy from another brand should be removed as much as possible first.)

If the epoxy is wet, soft or gooey like jello, it has to come out. Scrape or cut away as much of it as you can with a paint scraper or knife; denatured alcohol or acetone helps loosen it and evaporates to leave a clean, dry surface. Then pour a new flood coat for a smooth, hard, glass-like finish.

Epoxy that stays soft almost always means the batch was under-mixed or not measured accurately (1:1 by volume for UltraClear Bar & Table Top Epoxy), so measure and mix the replacement batch carefully.

There is no simple solution for a rubbery deep pour that never cured. Aside from physically scraping it out, some customers have had success with Klean Strip Premium Stripper, but results vary, it is a very harsh chemical, and you use it at your own risk, following the directions on the can.