Former tenant keeps collecting mail from new tenant's place!

[Jean Soo]

We currently have excellent tenants staying in one of our properties. However, the former tenant's mails keep coming to them so the former tenant keeps going through their mailbox to collect it. The new tenants had told me they didn't like this and we even tried telling the former tenant to stop doing this but they haven't listened. We updated the mailing address through UPS but no change has been implemented. Any action steps or suggestions? Thank you!