To delete a user's account from a Raspberry Pi, use the following command:
sudo userdel bob
and replace the username "bob" with whatever the username is of the account that you want to delete. When the user account was added to the Pi, a home directory for that user was most likely added, as well. To remove the home directory of the user, use this command:
sudo rm -rf /home/bob
and replace "bob" with the username, once again. After using
userdel and removing the home directory, the user account should be completely removed from your Raspberry Pi.
If you would like to add another user to the Pi, check out this post that details the process:Adding a User to a Raspberry Pi.