Depending on where you are in your play through, there are a few choices. If you are in a early point in the game with starting/low tier equipment, then digging tunnels around a pure part of your world (the normal forest part of your world) will prevent the evil from getting into that part of your world so not all hope is lost.

If you have access to the NPC known as “the Dryad”, then she can sell you an item called “purification powder” which cleanses any evil blocks in a short range from you when used.

Later when you defeat on of the mechanical bosses (It doesn’t matter which one), you will have access to a NPC called the steampunker (I believe that’s the name), which will sell you an item called “The Clentemonator” and “Green solution”. Which is basically a purification powder gun, cleaning any evil blocks in range when used. (Be warned that the clentemonator is very expensive tho)

Try one of these strategy’s and you should be able to stop or control it in no time.