@Richard466
Richard, you don't say whether you are prepared to accept another guest, you just say you have had problems in the past with 'visitors', so it is hard to advise you.
If you are prepared to accept another visitor as an official guest then I have to ask this. Is this stay currently in progess, or is it an upcoming stay?
If the stay is in the process of taking place you can say that unregistered guests are not covered by Airbnb protection scheme and you cannot take the risk of having an unauthorised guest stay on the property.
If the stay is yet to take place the guest, on their confirmation form, is required to put the names of additional guests and their email address if they have one.
When this is done they become an Airbnb registered guest, and you can advise the guest this additional person would be welcome.
If you simply do not wish to host an additional guest tell the guest that it is against Airbnb's policy to host unregistered guest, you are a Superhost and do not wish to do anything that may jeapordise your standing in the company.
Cheers.....Rob