Can't get rid of wdm

Question

I have a minimal installation of Ubuntu 14.04 with dwm as window manager.

The display manager wdm was installed as a dependency and is launched at every boot. But all I want is a manual login to X with startx (.xinitrc is configured with exec dwm).

• I disabled the service wdm using bum, but when I launch startx I got the error message

  Only root wants to run wdm
  

• The same error message appears after removing wdm completely with its dependencies.

• I also modified in vain the parameter splash quiet of /etc/default/grub to text.

I used to setup Debian and ArchLinux systems without having this problem. Is it specific to Ubuntu? What is the proper way to resolve my issue? I hope there won't be a confusion concerning the couple wdm/dwm ...