Why Remove Your Group From The LFM List?
Your party is partially full and you want to choose YOURSELF who is invited to the party, not have the LFG tool automatically add ANOTHER Hunter/Rogue/cloth-wearer because they checked off "Auto-Join".
Sometimes you've got enough people in your party that you don't need to fill it completely, yet you've listed yourself in the Looking for More tool, /lfm, and it doesn't auto-exit you until you have a complete party. If you went into /lfm and tried to set the dungeon name on the top right hand side to "none", you'd find you can only select a dungeon there, not unselect one.
Occasionally, someone is "just playing" because they haven't been a party leader in a group with friends to check it out - and then they don't know how to remove the party afterwards.
How To Remove Your Group From The LFM List
It's simple - go into /lfm, and where it says "Dungeon" on the top left hand side, select the drop-down and choose "None".
This instantly removes the party from the queue and listing.
The only requirement is like putting a group INTO the LFM tool, removing a group must also be done by the party leader.