Rule Definition
Unreferenced code may be the symptom of Dead Code. Dead Code must be avoided as it makes source code less readable and increases the cost of the software maintenance. Warning: Unreferenced code can also be the symptoms of missing code (code not included in the source code analysis) and / or can be the symptom of use of polymorphism. The intensive use of polymorphism can make the application easier to change but much more difficult to test, debug and stabilize.

Remediation
Check if the Interface is truly unnecessary. Remove Interfaces that are not used.