That's not easy, as the OnFetchTypedList is called prior to the execution, so whatever you do, it simply proceeds.
You have to override ExecuteMultiRowDataTableRetrievalQuery as well. You should set a flag in the DataAccessAdapter derived class (a boolean private field) when OnFetchTypedList is called, and in the override of ExecuteMultiRowDataTableRetrievalQuery, you should check the flag. If it's true, simply return, otherwise call the base' method.