You could try running your server with online mode (which means you might have to purchase a copy of Minecraft) so that players have to authenticate (proof that they're themself) themselves before they can join.
Otherwise, what you'll have to live with it, and ban yourself (to stop others from using your name) and deop yourself whenever you log out.
To set your server to online mode, go to your server.properties file and edit:
online-mode=false
to:
online-mode=true
Either that or change your account's password and secure your email address.