Here's the big question: Why do you want to stop it from spreading content? The purity is very lackluster. Exceptions include corruption/crimson approaching your base of operations (also where your NPC's live). Solutions to this are just live in the sky, or dig out a tunnel around your base so nothing can spread there.
Ultimately, it is best to be in the spreading biomes as much as possible to get the most out of their content, specifically acquiring important items such as the key molds and Crystal Shards.
The Jungle can be threatened, however I emphasise that word strongly. You can make preparations to protect some of the jungle if some evil chasms have bunkered into it already, however there is no real need to do so as the surface Jungle is also lackluster in content as well as the fact it takes a long time for it to spread deep down. Small worlds are affected the most, though I never really advise playing on small worlds because of how messy they can be (unless you want to see worldgen cock-ups).
Realistically, concerning the spreading biomes, on many play-throughs I have done on large worlds, you need not bother tampering with them as I always see plenty of jungle left and spots of purity still remaining long after I've reached end-game. Except a quick 5min alteration to my base perhaps, if the spreading biome spawns close to me.
I'd advise that you just enjoy the game and not worry about what others get worked up about. Completely preventing the spread of biomes, especially with the Hardmode strips, can take a few hours of menial work. That's not something you should be doing in a game like this.
