First, it's important to understand whether your bank uses method called "OAuth" with Mint. If they do, it means that rather than entering your username/password on Mint, you will instead get a pop-up window that will take you to the banks’s site and ask for your credentials. The bank will then give us a token that we use to be able to connect to your bank account.
What do I do if my bank uses OAuth and I suspect someone else is accessing my bank account?
If you think someone else might be accessing your account, please go to your bank's website to change your password AND revoke Intuit’s token. When you come back to Mint, you'll see a connection error. At that point, you can safely re-add the account the way you did before and you will now be the only one with access.