Dried epoxy resin can be pretty hard to remove. However, as epoxy in general is a difficult substance to deal with, the removal process actually should take place after the epoxy has dried, but before it has cured. The curing takes place after the epoxy has dried in the hours and days following the application. As a result, you should allow any spilt epoxy to dry completely, but then clean it immediately.
Put on your gloves. Paint thinner and denatured alcohol will not hurt you, but they are not good for your skin. Protect yourself with gloves and avoid any contact with your face or eyes.
Pry as much of the epoxy off the surface as you can. Dried epoxy should flake or chip off in chunks. Use the putty knife to remove as much as possible and create a flatter surface.
Saturate the lint-free rag with the denatured alcohol. This will usually be sufficient for recently-dried epoxy. Epoxy that has started to cure (has been dry for more than about 24 hours) may require paint thinner to soften it up.
Use the lint-free rag to wipe away the epoxy. This will not be easy. You may have to scrub and pick at the dried epoxy and reapply the paint thinner or alcohol multiple times. The epoxy should start to loosen up and chip or scrape off as you continue the removal process.
Wipe down the entire area. Once all the epoxy is gone, you will need to wipe down the entire area with a clean rag. You can use water or a cleaner appropriate for the surface that you are cleaning.
Paint thinner and other solvents can damage some surfaces. Make sure that you have tested them on an unnoticeable area before you apply them to the surface with the dried epoxy. In some cases, dried epoxy may be the lesser of two evils.