Cannot delete domain login from SQL SERVER 2012
Check to see if they own the endpoint itself:

SELECT SUSER_NAME(principal_id) AS endpoint_owner ,name AS endpoint_name
FROM sys.database_mirroring_endpoints;


If so, you'll need to change the endpoint owner. Say the endpoint is called Mirroring, and you want to change the owner to SA:

ALTER AUTHORIZATION ON ENDPOINT::Mirroring TO sa;