Currently it is not possible to hide the competencies displaying in Coaching Advisor page because this is a system behavior that cannot be changed.
SAP SuccessFactors Succession & Development
As it's not possible to hide a specific competency set from dropdown in Coaching Advisor, in case you consider this an important enhancement for your business, you can share this idea in our SAP Customer Influence & Adopt page so it can be considered to become a future enhancement.
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SAP SuccessFactors Succession & Development all versions