There used to be an option in the interface somewhere to turn off XP gain, that's how twinks stay at a certain level.
There should be NPCs that let you lock experience in Stormwind and Orgrimmar - look for the title <Experience Eliminator>. Behsten - the War Room, off Stormwind throne room. Slahtz - the top floor of the Hall of the Brave at the far end of the Valley of Strength. And having written them out like that I've just seen the awful pun for level-capped twinks.
My lvl 87 mage is stood in front of Slahtz right now and has the option there. What level is your toon?