To find out if you have any PPAs, open a terminal, and type
sudo bash, followed by your password. Then cd to
/etc/apt/sources.list.d and type ls | grep libre*. If you see any results in the terminal from this, you probably have a LibreOffice ppa, which you can delete using
rm (whatever results the previous command gave you), or by using
rm libreoffice*. However, it's worth noting that LibreOffice is also available from the Ubuntu Main source, which you shouldn't remove.
As for any LibreOffice trying to reinstall/upgrade itself, Ubuntu will only ever upgrade packages that are already installed. Just because you are subscribed to a software source, doesn't mean the software in said source will be installed. Odds are, you have some remaining peaces of LibreOffice left.
I would first try running
sudo apt-get autoremove, and see if that helps. The
autoremove command tells apt-get to remove any unnecessary packages from your system, some of which may be LibreOffice-related. However, if apt-get thinks some of the packages are important, it will not remove them, as may be the case here.
You can probably test this using
sudo apt-get purge libre* -s. Do note that the
-s at the end tells apt-get only to simulate the removal process: nothing actually gets changed. However, you will see a number of apps which the system pretends to remove, in which case you should be able to pick out the ones that actually seem LibreOffice-related (be warned: many apps and libs will probably be unrelated; use caution when removing them, as they may be important to your system) using
sudo apt-get purge (program). From there, you can run
sudo apt-get autoremove like I mentioned above, and that should get rid of LibreOffice completely.