De man sshd_config
AllowUsers
This keyword can be followed by a list of user name patterns, separated
by spaces. If specified, login is allowed only for user names that match
one of the patterns. Only user names are valid; a numerical user ID is
not recognized. By default, login is allowed for all users. If the pat‐
tern takes the form USER@HOST then USER and HOST are separately checked,
restricting logins to particular users from particular hosts. The
allow/deny directives are processed in the following order: DenyUsers,
AllowUsers, DenyGroups, and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
Entre. por favor não permita senhas de login para o seu usuário root. Permitir apenas chaves ssh ou, melhor ainda, permitir que um usuário específico faça login e mude para root, mas não para root diretamente.