My friends and I played on this server I made. One of my friends would never wear diamond armour, he would only wear iron. So I made an army of armour stands with all diamond armour using HIS diamonds that he wasn't using.
Not bad, considering that one machine can turn them back to diamonds if needed (in the top of my head at the moment is the rock crusher from railcraft but I'm quite sure there's other options too)
I need to start a topic for pranks, maybe some people will get some ideas out that could be fun to use (and not deadly ideally)