You can use RadDiagram.ConnectionManipulationCompleted
event to achieve your requirements. In the arguments of that event you will be able to access all the information that you need to decide if the connection should be attached to the target or not. If you handle this event the connection will not be generated. In your case you can check the shape geometry and handle the event when the shapes are Ellipse and Hexagon. You should only have in mind that depending on the action the target or the source of the connection could be not set yet and then you can use e.Shape from the event argument which represents the new value (e.g. if you change the target the e.Shape will provide information about the target, if you change the source - for the source).
For your convenience I implemented this approach in the attached project. Please take a look at it and let us know if it works for you.
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