Because Viva Engage is a private tenant just for your organization, it's designed so that coworkers can follow other coworkers who provide information that helps them do their job. This means that you can't block coworkers from following you in Viva Engage. However, there are several ways that you can limit who sees your messages:
Send private messages only to those users with whom you want to communicate.
Create a private group, and invite only those people with whom you'd like to interact.
Create an external tenant, and restrict invitations to admins only.
Note: This applies only if your tenant allows for external tenants.