What's the most efficient way to get (rid of) experience?

/xp -5000000L mikeTheLiar

The xp command does not work with negative experience points, but works for negative levels, according to the Minecraft Wiki.

You can just /kill yourself.

You will probably want to learn how to use the /tp console command to go home so you can drop things off first though, to avoid losing equipment. (I always found the lack of built in warping to be unsatisfying.)