Is it possible to completely disable TLS?
Yes, it is possible to completely disable TLS.
However, you need to understand that FileZilla has updated their FTP client and "FTP over TLS" is now used by default. This may result in your clients being unable to connect to your server if you completely disable TLS. https://filezilla-project.org/versions.php
You should upgrade to Serv-U v.15.1.2 as there are more granular options to disable SSL and TLS. (See Attached Screenshots)
Global > Limits & Settings > Encryption Tab
To test please use FTP Voyager instead of FileZilla because of the FileZilla default is set to use TLS.