You can create a custom field for experts (both freelance and corporate), where you will specify if the expert is unavailable. After this, the experts list can be filtered via the Local Custom Filter to show only experts with a certain value of the custom field.

Custom fields can be created on the Fields section of Corporate Settings tab of Projetex Server Administrator.
You will be able to see/change custom field values from the Custom tab in the Freelance/Corporate Expert profile (Main tab of Corporate Expert / Freelancer window).

For more info on creating custom fields, please refer to this article: https://aithelp.com/Knowledgebase/Article/View/650/0/how-can-i-create-a-custom-field-in-projetex