Hi Daniele, 

For setting grey option in the drop down you could use a css to do so, based on an additional field from the drop down source. 

In case of click of one of this options you could add logic to ignore the selection reseting the variable associated to the drop down. 

Hope that it helps you 

Hi Alexandre, thank you for your support.

It is no really clear to me how to associate the css to a single option of a interaction\dropdownsearch block and how to access the additional field of the source.

Are you sure you are talking about the interaction\dropdownsearch block and not about the dropdown widget?