If you chose to define your own join paths, use the RemoveTopic() method of the DefinedJoinPaths (Collection) to remove a topic based on an item on the Request and Limit lines. This method corresponds to selecting all available topics on the Define Join Path dialog box and removing the values in the Topics in Join Path list.
ActiveDocument.Sections["Query"].DataModel.JoinsOptions.DefinedJoinPath["MyJoinPath"].RemoveTopics(String DefinedJoinPathName)
In this example, all topics are removed from the MyJoinPath join path:
ActiveDocument.Sections["Query"].DataModel.JoinsOptions.DefinedJoinPath["MyJoinPath"].RemoveTopic("Products")