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I don't recall exactly what I did to downgrade all the packages I got grom backports, but it may have involved uninstalling a bunch of stuff (including xorg) and then reinstalling. You can see what you have installed from backports with

dpkg -l | grep bpo

I had the same problem with the system being spammed full of backports.

After checking for packages named bpo as above and disabling the backports repository I did the following:

apt-cache show pkgname|grep Vers

From here I just chose the latest version that did not include "bpo" and used apt-get to downgrade the packages individually.

apt-get install pkgname=pkgversion

Where pkgname is the package you want and pkgversion is the latest one returned by the previous command. Where possible I removed and reinstalled packages that wouldn't clobber the system and downgraded the rest. It's a little time consuming, but everything turned out OK.

Until then, you can prevent this from happening by pinning backports to a lower priority. Put the following into /etc/apt/preferences.d/00backports

Package: *
Pin: release a=jessie-backports
Pin-Priority: 100

This is a lot easier to prevent than it is to fix it. I don't recall exactly what I did to downgrade all the packages I got grom backports, but it may have involved uninstalling a bunch of stuff (including xorg) and then reinstalling. You can see what you have installed from backports with

dpkg -l | grep bpo