SOMEBODY KEEPS HACKING MY MINECRAFT SERVER? HELP!?

Somebody keeps hacking my Minecraft bukkit server and my friend's account. He knows my friend IP address. He appears when my friend login to my server but when my friend and I go to other servers it doesn't happen. He keeps annoying us and our players. I changed my IP address and I opened another server on my other computer but he can still hacks. He hacked my friend account, when my friend login he will use my friend account to chat and the weird thing is my friend and he can both online using the same account. He changed the password but it didn't work. I don't know what is happening.