Does anyone know how we can switch off FTP functionality for Centova and return FTP features server-wide, as it was before we installed Centova?

If you're just asking how to remove Centova Cast's FTP integration, you just undo the steps you performed in section 2 of the installation manual.  If you let Centova Cast set up FTP integration automatically, it would have made backups of your ProFTPd configuration as /etc/proftpd.conf.pre-centovacast or your Pure-FTPd init script as /etc/init.d/pure-ftpd.pre-cetnovacast which you can restore from.

If, OTOH, you're saying that after performing FTP integration your other FTP accounts stopped working, that means FTP integration was performed incorrectly.  (Possibly you have safe_mode turned on or open_basedir restrictions enabled, etc.)  When done correctly Centova Cast's FTP integration will work alongside your original FTP accounts.