Thanks, I just did a hack by placing a absolutely positioned TextBlock Text ="And" on top of the logical operator so that the user can't click on it :-)
i have a similar requirement because I am using the RadDatafilter to create a "set" clause for an update query, and I just used a hack of
if (filter.FilterDescriptors.LogicalOperator == FilterCompositionLogicalOperator.Or)
filter.FilterDescriptors.LogicalOperator = FilterCompositionLogicalOperator.And;
to reset the value to 'And' in the eventhandler for FilterOperatorsLoading.