2) Go into your FTP and then your world folder, then into the playerdata folder and delete the file that has the same name as that player. Note: This will clear all of their items.
To permanantly fix the issue:
1) Identify where the player is, you can do this by doing /seen <playername> and it should give you the coordinates.
3) With the region file name (r.0.0.mca for example), go into your world folder and then region folder and find the file with the same name and delete it (Note: This will remove that area).
You can also attempt to use something like MCEdit or another Region Fixer.
