If you want to add or remove a registration from a user (registrations are how a user is authorized to use an application) you could either allow someone to use the FusionAuth admin screen (possibly with a limited role) or you could use the API to build your own more limited interface.
Here's the API call you would want for this specific use case: https://fusionauth.io/docs/v1/tech/apis/registrations#delete-a-user-registration