How To Delete Player On Your Minecraft Server

Preparation

Stop your game server before edition files.

Players data folder

Go to your Game server > File manager page, open folder world/playerdata

Delete player data

You have to encode the player username to UUID - https://www.google.com/search?q=minecraft+username+to+uuid&oq=minecraft+username+to+uuid
For username "scalacube" it is 57dcb79a-6bfb-414d-b94a-ed8b628b4485.
Delete file 57dcb79a-6bfb-414d-b94a-ed8b628b4485.dat.