The following error will be thrown when trying to delete SmartObject where SmartObject properties are not the same as ServiceObject properties:
Inconsistency detected between SmartObject '[SmartObject Name]' and Service Object '[ServiceObject Name]' during loading of SmartObject Definition. Property : '[Property Name]' not found in Service Object '[ServiceObject Name]' Definition.
In the above example, editing SmartObject and removing SmartObject properties that no longer exist in the Service Object, save the change, will allow SmartObject to be deleted.
Alternatively, adding the missing properties (columns in SQL table in this example), and refreshing Service Instance (SQL Server Service in this example) will allow SmartObject to be deleted as well.