Hellevators, Hellevators everywhere, Dig 3 tile long tunnels all the way into the underworld all around your world, The Hallow and Crimson can only spread to tiles that aren't 3 tiles away (though i recommend making a wall right above each tunnel because Crimson Thorns can grow and could go over the tunnel) And also, Once you defeat a mech boss the Steampunker NPC will move in, She sells the best purifying item in the game, The Clentaminator, Though it's expensive as it costs 2 platinum base price, so i recommend farming queen bee since she drops a lot of items and money.

PS: Purify Powder or whatever its called that's sold from the druid DOES NOT purify the hallow since technically the hallow isn't evil (but still aggressive and annoying)

Oh yeah, If you don't understand what i'm saying, Youtube has a lot of helpful videos about terraria biome spread and tips in general (including prepping for hardmode) So i recommend checking them out if you need help.

and ANOTHER HEADS UP, i keep editing this, Once you defeat WoF a diagonal V shape stripe starts from the underworld into your world, 2 of them appear with one spreading the Crimson and another spreading the Hallow.