If you are logged into the client machine, right-click on the avast icon in the system tray and enter the password (which is configured in ADNM -- right-click "Computer Catalog", choose Properties, and select Service under "Policies: avast! Antivirus").
Then a context menu will appear and you can choose "Pause Provider" or "Stop Provider" to stop individual services (probably you only need to disable "Standard Shield"), or you can "Stop On-Access Protection" to completely disable Avast.
You can then use the same context menu to re-enable the provider or Avast in general, or just log out of the machine. The next machine a user logs in Avast will start up as normal.