Hi,
I would like to suggest you to use the SetPropertyCmd inorder to make the CustomSymbol to be inactive when added onto the Diagram.
The SetPropertyCmd sets the value of the property in one or more nodes.
I have created a sample that would help you to implement this.
Kindly, refer the Sample and let me know if this helps you.
Thanks, for your support to Syncfusion.
Regards,
Praveena.
ControlNodeSample.zip