"Specific People" showing up as guest user on SharePoint

If you share documents or folders via individual links by Share > Link settings > Specific people, the external user will not be added to the organization directory and will have access to the shared objects via the sharing links only.

Sharing a list/library or a site to external users is a different process. The external users will have to accept the invitation sent via Email and then sign in/register with a Microsoft or Organization account in order to be added to the organization directory, or as you put, to become a “guest user”.

You can remove the guest users from this tenant via Microsoft 365 admin center > Guest users > Delete a user. It does not have influence on the links you have already shared in SharePoint lists or libraries.

Reference: External sharing overview.