How to prevent users from dragging or dropping the row or column headers in the grouping bar?
It can be done by setting the property AllowRunTimeGroupByField for the pivot item to False.
<syncfusion:PivotItem FieldMappingName="Product"
FieldHeader="Product"
TotalHeader="Total"
AllowRunTimeGroupByField="False"/>
this.pivotGrid1.PivotRows[0].AllowRunTimeGroupByField = false;