Old slug still available

Hi, I installed the plugin and works fine. Now I see the new coded slug and help me a lot. But there’s something still to fix. I checked that if I visit the old slug (with the real name) it’s still working and redirects to the coded one.
This means that if someone try something like /johnsmith will provably get to a real person, even if this person have the official slug coded
What I need is to disable the old slug.
How can I do it?
Thanks