The short answer for removing cured epoxy from a surface is removing it mechanically, but there are a few different methods to consider. It is strongly recommended to choose the safest method possible. Take extreme care not to damage the substrate when removing cured epoxy.
Some surfaces that reject epoxy naturally, such as some plastics, silicones, or similar materials, are much easier to remove epoxy from. Flexing the material or a gentle tap with a chisel or similar tool can usually dislodge a piece of cured epoxy.
To remove cured epoxy from non-porous substrates that epoxy forms a good bond to requires more effort. Sanding or grinding are usually the most common methods.
For porous materials, such as wood or concrete, place a rag that is dampened with acetone around the cured epoxy; the acetone will work into the substrate around the epoxy, and will allow it to be pried up eventually.
Another method for some substrates is to use a heat gun. Epoxy can only tolerate so much heat before its physical and bond properties begin to weaken. A putty knife or chisel can peel up the epoxy once it begins to weaken. Take extreme care to only use this method when it will not cause any damage to any of the surroundings, and only use this method if it is safe to do so.
Some chemicals such as TotalBoat Eco Solvent, heavy-duty adhesive removers, or heavy duty paint strippers can also remove epoxy if the epoxy and the immediate surrounding area is in contact with the chemical, but this method is usually very slow, and can potentially do more harm to the substrate. The chemical needs to remain in a liquid form and can not be left to dry out. The undesired epoxy can eventually be pried up or peeled back with a putty knife. Extreme caution should be taken when handling any of these chemicals.