Hi, and welcome to Zorin OS!
Have you uninstalled the "old" version of LibreOffice? If not, I would recommend you do just to avoid having duplicates of the same software.
The launcher entries that you see in the applications menu are located in /usr/share/applications. You can navigate to this location using Nautilus (the file manager) by pressing Ctrl+L and entering this location at the address bar. There, you will find a lot of files with a .desktop extension.
I don't have LibreOffice installed myself at the moment, but it should be named something like libreoffice-writer.desktop, libreoffice-calc.desktop, and so on.
In order to prevent LibreOffice from appearing in the applications menu, you can rename them so that they don't end with .desktop. Note that these files are owned by the root user, so you'll need to launch Nautilus with elevated permissions.
Renaming them to something like libreoffice-writer.desktop.bak should be enough. This way the files are still there should you ever need them back. You might need to restart to apply the changes.