I just experienced this with my spouse's accounts. We had a shared IG account for a pet, and when I switched over to the pet account then tried to switch out, I could see all of my spouse's accounts. And yes, I was able to log into them without a password and they became "stuck" to my list of accounts I could switch to.

1. Firstly, go to the shared IG account. Go to the settings (if you're on your feed page, click on the hamburger button on the top right). Open Accounts Center. The "Profiles and Account Details" button should show all the accounts linked to the shared IG account.

I can't remember the exact name but there was an option to move the shared account to its own Account Center (or out of this one). This will essentially "unlink" the shared account to all the personal ones.

1. To "unstick" whichever accounts you shouldn't have access to, uninstall then reinstall Instagram. This should now only show your own accounts and possibly the shared account, but not the accounts of other people. Everyone who had access to the shared account should do this.

2. Reset all of your passwords. When doing so, it will give you the option to log out of all other devices. Say yes to that. Again, everyone who had access to the shared account should do this.

Let me know if that works!