If a user reject sharing his/her data, which are mandatory for the destination service, what should we do?

[fishfree]

I mean the data are mandatory for using the destiniation service. Are there some best practices for this senario?

[perki]

The sharing is a "contract" between the user and the requester.
By rejecting sharing his data with a service that depends on it, the user explicitly refuses to use the service.

Having some users "refusing" is OK as long as they represent a small set.. If they are too much, the most important is to review the "consent message" and the "scope of data" to motivate the user to accept the sharing.