I had to go looking for this the other day so I'm posting it here for future reference.


Type Set-BrokerAccessPolicyRule “DesktopGroupName_Direct” –AllowRestart $false (Replace “DesktopGroupName” with the name of the desktop group that you wish to disable this feature for), press Enter.
Type Set-BrokerAccessPolicyRule “DesktopGroupName_AG” –AllowRestart $false (Replace “DesktopGroupName” with the name of the desktop group that you wish to disable this feature for), press Enter.
Check results of commands by typing: Get-BrokerAccessPolicyRule and press Enter.
http://support.citrix.com/article/CTX131921



Outside of Powershell you can go to: Note: This option is available only for VM-based desktop groups.

You can configure desktop groups to allow users to restart their own desktops locally if they fail to start or take too long to connect. User-driven desktop restart may result in loss of data;ensure that all users who have access to this option are aware that their work is not saved if they select to restart their desktop.

In the Delivery Services Console tree, select the group for which you want to configure user-driven desktop restart.
From Common Tasks, select Enable user-driven desktop restart. If user-driven desktop restart is currently enabled, the Disable user-driven desktop restart task appears instead.