Note: You need to stop your server before making the following changes
Step 1:
Log into your game panel and click 'stop' on your service. (This is very important)
Step 2:
Select the 'Configuration Files' icon from the control panel

Step 3:
Edit the 'users.cfg' using the text editor
Step 4:
Find the user having issues and delete their entire line
Step 5: (From here on the process gets complex)
Log into your service via FTP.
What is FTP: https://www.streamline-servers.com/billing/knowledgebase/5/What-is-FTP.html
How to Access FTP: https://www.streamline-servers.com/billing/knowledgebase/29/TCAdmin-FTP.html
Step 6:
Navigate to /saves/yourworldslot
This can be found in the serversettings.cfg
Step 7:
Find the users STEAM 64ID.
You can use a program such as: https://steamid.io/
Step 8:
Search for all files containing the STEAM64ID and delete them (this can take a little awhile).
Step 9:
Start your server