Are you using KDE? I had this problem the other day, and
systemctl stop/disable packagekit didn't help at all.
Here's the prompt:
In this example, polkit.subject-pid is PID 2201, which is:
username 2201 0.0 0.1 1354816 24440 ? Sl Oct27 2:46 kded5 [kdeinit5]
Which suggests that KDE might be doing something. On my system, KDE doesn't have package management settings in the system settings tool, but opening apper's settings menu I found this:
Setting this to Never took care of the problem for me.
systemctl mask packagekit works as well. See for example http://0pointer.de/blog/projects/three-levels-of-off, which describes the difference between systemctl stop, disable, and mask. mask makes services completely unstartable until they are