How can I preserve a Jungle that was generated with Corruption inside it?

In order to save your Jungle, you need to act quickly to contain it completely. Dig a gap that is at least four blocks wide surrounding the entire corruption. Note - on the bottom side, Corruption will send down hanging vines that will spread corruption, so you must also line the bottom with a non-corruptible material like Wood.

If it's not feasible to contain all the corruption this way, the best you can do is isolate some parts of the Jungle and Underground Jungle with a four-block wide border, so you have at least some jungle to work with until you get a Clentaminator and enough Green to cleanse the corruption - which will allow the Jungle to spread to those tiles again.

If you dig a 4 block wide empty space around the entire corruption, then it shouldn't be able to spread any further