If you have been skulled, all you can do is avoid re obtaining one in combat, and in about 21 minutes it will go.
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You can appeal it on the RuneScape site.
The Left Skull Half is dropped by Ankous on the fourth level of the Stronghold of Security.
Your skull will go away after spending enough time away from a PVP area.
You walk around in a non pvp world for around 10-20 minutes
If you are thinking of the skull on top of your head, that means you have done pvp in the wilderness or the pvp selected worlds. You can't get rid of it manually, you have to wait 20 minutes for it to go away. Also the skull means that when you die you lose everything, to prevent this press the backpack on the prayer menu.