Change PvP Settings

Before following these steps, make sure to stop your server first. This is to avoiding breaking anything on your server while you edit. 
Step 1: Head to your game panel and look for your File icon on the left hand side.
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Step 2: When you are in you files, scroll down until you see a file named: serverconfig.xml
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Step 3:  Open the file and look for the line that says: <property name="PlayerKillingMode" value="3" /> . When you have found this line, you just need to change the number in the Value area. 
Before:
<property name="PlayerKillingMode"                value="3" /> <--- Everyone can kill each other
After:
<property name="PlayerKillingMode"                value="0" /> <--- No one can kill each other
There are 3 settings for Server Visibility ranging from 0 to 2.
Each number has its own setting:
0 = No Killing: Players cannot damage one another under any circumstances.
1 = Killing Allies Only: Players can only damage each other if both agree to it by adding each other to their Allies list using the in game players menu.
2 = Killing Strangers Only: Players can damage each other UNLESS they are Allies.
3 = Killing Everyone: Players can damage everyone, regardless of Allied status
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Make sure to save before exiting the file.
After Completing these steps, you can now go and start you server.
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