Mastering Windows Group Policy

Preferences stick around after the GPO is removed

And, once again, always remember that both unmanaged policies and true preferences within Group Policy are not self-regulating, but rather "sticky" inside the registry. If a GPO applies settings to a machine and an administrator then tweaks settings inside GPMC so that the GPO no longer applies to that same machine, anything that is a true managed policy setting will be automatically removed from the workstation, but anything that is a sticky preference will hang around until changed by the user, or until flipped back via a new GPO.

Both of these data points apply whether we are talking about Group Policy Policies that are unmanaged, or if we are talking about actual Group Policy Preferences. ...