How to change other users' nicknames in multiplayer?

Question

How can I change nicknames of people on other servers?

Answer 1

Short: You can't.

Long: You can if it is your server. If you have f.e. Bukkit installed, you can install/write a plugin which will allow you to temper with the names of the players. This brings two problems:

1. Only the chat-name can be easily altered.
2. If you want to alter the name above the head, all settings of that user will be moved to a file with the name...which pretty much renders it useless.

If it is not your server, you can't.