How to prevent enemy spawns in an artificial jungle biome?

Solution 1:

If you want to prevent enemies from spawning in a Jungle biome (artificial or not), build several houses and have NPCs move into them. This is the only reasonable way to fully stop hostile mobs from spawning in general.

Note that during Blood Moons enemies will still spawn, even with NPCs nearby.

Solution 2:

From my experience in playing with Terraria, when you create an artificial biome, the game will treat it as if the biome "exists" to begin with.

The problem with walling your artificial biome is that, the jungle seeds will be unable to "grow". The artificial biome would grow the size you want it to be.

Here is a guide about gardening that could be relevant to you:

http://terraria.gamepedia.com/Guide:Gardening

If you notice, the objects were grown according to how they are found in the wild, while creating an enclosure to make it easier on you to get the items.