In the file manager under Saves and your world, there is a Players.xml file and a Player folder. In there, if you find the entries relating to the specific player's Steam ID, and delete them that should remove the specific player's data for you.
1. Go to your Gamepanel and open the file manager.
2. Navigate to Saves\YourWorldName\YourSaveName\Player
3. Find <PlayersSteamID64>.ttp and <PlayersSteamID64>.ttp.bak
4. Delete both.
5. Go back to the Gamepanel and start your server.
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