Comment # 1 on bug 1222088 from Michael Andres

May very well be that some other package required or recommended libreoffice,
and that's how it got selected. Just deselecting a package does not prevent
it's installation at a later time. For this, a lock must be applied.

Locking an installed package prevents it's removal or update, locking a not
installed package prevents it's installation. If you're using Flatpak
libreoffice, you probably want to lock the libreoffice package.

Try to remove the libreoffice packages again:
> zypper rm --clean-deps libreoffice

Then apply the lock:
> zypper addlock libreoffice

Check with:
> zypper locks