Minecraft offers varying levels of difficulty, each suited for a different play-style. The difficulty setting can change various aspects of the game, including the amount of damage dealt by mobs, as well as the way hunger and poison affects players.
Changing the Difficulty
On the left hand side of your NodePanel, click on the "Game Settings" section.
Then select the "Gamemode" tab near the top of the Panel. Here, you will find different option boxes you can select of the various difficulty options, including Peaceful, Easy, Normal, Hard, and a checkbox for Hardmode as well.
Select which option you prefer, and Save the changes. Your server will now be running at this difficulty when you have issued a server restart.
Setting the difficulty to peaceful will forcefully despawn any hostile mobs from the world, and prevent any from spawning further.