Hello Jeff,
You'll have to change the cascading behaviour between your Opportunity and Task/Phone Call/E-mail/... To do this, go to Settings - Customizations - Customize the System - Collapse Entities and open your Opportunity Entity. Go the the 1:N Relationships (One
Opportunity can have multiple ...). Look for a relation where the related entity is Task/Phone Call/E-mail/Appointment... and open it. All these relations have a Parental Type of behaviour which means when you Assign, Share, Unshare, Reparent, Delete or Merge
the Opportunity record, the sub records will me Assigned, Shared, Unshared... too. To remove this behaviour, change the type of behaviour to Configurable Cascading and change the 'Assign' action to Cascade None which means: When an Opportunity is assigned
-> Do Nothing wiht the related records .
More info about cascading behaviour: https://msdn.microsoft.com/en-us/library/gg309412.aspx#BKMK_ReparentAction
Cascading Types: https://msdn.microsoft.com/en-us/library/microsoft.xrm.sdk.metadata.cascadetype.aspx
Hope it helps!
/J
