Se um contato / contact_group não for definido pelo serviço (ou qualquer um dos objetos pai), as informações de contato dos hosts serão usadas.
Portanto, a resposta simples é simplesmente garantir que as variáveis contacts / contacts_group NÃO sejam definidas para esse serviço.
Normally you have to either explicitly specify the value of a required variable in an object definition or inherit it from a template. There are a few exceptions to this rule, where Nagios will assume that you want to use a value that instead comes from a related object
Object Type Object Variable Implied Source
Services contact_groups contact_groups in the associated host definition
notification_interval notification_interval in the associated host definition
notification_period notification_period in the associated host definition