I thought I'd post this for those that don't know and might want to.
Combat XP can be disabled by right clicking on the xp bar and selecting (drum roll please)disable combat xp.
Note: This does not disable quest xp, only xp gained from killing things.
Reasons you might want to disable xp:
1) Quests that you can no longer access if you level to high, example: Betrayal Quest
2) You want to be able to get chests from mobs before they turn grey and no longer drop them, example: The Sand Shard quest on IoR.
3) You want to let some friends catch up to you, but you still want to kill things for loot and titles.
4) You're masecistic(I do not know how to spell) and want to level only through quests. (Admirable, but insane)
Reasons submited by others:
5) You want to max weapon skills. (courtesy of Prometeus Vaer)
6) Farming for tradeskills and wholesaler quests where you don't want your adventuring level to go higher than your artisan level. (courtesy of Baron von Devilwind)
Anyhow if anyone has any other reasons feel free (as if I could stop you) to post.
Thanks to those who added some reasons. I should further note that you cannot disable Artisan xp.
Edited, Mon Nov 22 21:29:28 2004 by Shardangel