Tell them that you do not allow guests to receive mail at your address. Recommend that they can get a post box at the USPS, the UPS Store or Fedex Store. Update your house rules and Things to Know section to state this also.
@Patrice38 I provide a link to a private mailbox service - for example in the US, you have companies like MailBoxesEtc. Any host considering long-term bookings should make an effort to be aware of the services that might be useful to long-term guests, but it doesn't necessarily mean that you have to provide them.
@Debra300 @Anonymous The amazing USPS has you covered. You do not have to pay for a post box. You don't have to pay for a service. You can just have mail addressed to
Your Name
General Delivery
City, State Zip
Then you just have to stop by the local PO and pick it up. Easy and free.
Especially now - Americans, support the USPS! And don't forget you can always send live chickens through the mail.