Product Family: Act!
Product: Act! Pro, Act! Premium, Act! Premium (access via web), Act! Premium Cloud
Version: 2005 and later
There is no setting to deactivate the alarm for activities that have already been scheduled, but it is possible to amend the activity preferences. So that future activites are set to not have an alarm. For these settings please see the following knowledgebase article:
How to set default preferences for scheduling Activities in Act!