In theory you could unzip Game.eco, delete your user file. Then zip the folder and rename it Game.eco again.
But when I do it it's not the exact same compression and the server don't recognise the files anymore.
this is very close. So i have had to delete users on my server before. What you need to do is navigate to the game.eco file (save a copy somewhere for backup reasons) located in the Storage folder. Then RIGHT click on it and OPEN ARCHIVE (for this you will need something like 7zip) then navigate to the USER folder. locate the player name, and delete it. do not unzip or rezip, simply close the 7zip program and place the game.eco folder back into your Storage Folder.
I have uploaded images to help. good luck mate!