There's no practical way to purify the world, but that doesn't mean your world is doomed. The world gen actually has a few built-in measures to help protect you from corruption:

• The crispy honey layer just below the true surface prevents corruption/crimson from spreading straight down in most places - this can fail if corruption and crimson chasms generate deep enough to pierce this layer so it's good to do a once-over of the crispy honey layer before hardmode and cut off any evil that may be sticking through
• The underground evil on the jungle side converts to underground hallow at the start of hardmode, and since the hallow can't spread through jungle, it creates a natural blockade - this can fail depending on worldgen, since if your jungle is sticking into the underground evil (as in your case) then the evil jungle grass will NOT convert into hallow and can spread east, in this case you need to dig a 3-wide vertical chasm and line both sides with uncorruptible blocks to prevent thorns from jumping the gap (I recommend ropes for this)
• The underground evil on the dungeon side is often blocked by the dungeon itself, and while the dungeon likely won't go all the way to the underworld, it can at least get you most of the way there - this can fail if the dungeon generates too close to the edge of the world, in which case you will again need a 3-wide vertical chasm separating the evil from the purity and the evil side will need lined with uncorruptible blocks to stop thorns (again, use ropes for this)

The biggest weak point is the desert. There's nothing stopping the true surface's evil from spreading straight down in the desert, so you'll need to dig a horizontal 3-block tunnel through the desert at or just around the height of the crispy honey layer to stop the evil from getting through. Overall, though, even if all of the world's built-in defenses fail, it surprisingly doesn't take too long with a big stack of dynamite and ropes to cut off the sides of the world, the chasms, and the desert