admin password

we are trying to figure out how to make sure no one who were given a login and password can access our wordpress account.

Basically, the question is: how can we make sure no one else has access to the wordpress back end of our website, including our web guy who is the only one who has the admin login and password, but does not work with us anymore. we want to make sure he does not “sabotage” our site by accessing it… He gave us few month ago access to the site but not the main admin account. is there a way we can access it anywhere or delete it?.