There are several good answers already about if you should do this or not with or without being paid.
My answer comes from more of a security perspective and assuming that you'd rather spend some time to get them off your back and move on with your life. There is no way I'd let a former employer know any of my credentials particularly if there was bad blood.
What I propose would take some time. If you have been paid for the final week, consider it as your final task to be completed at home. If you are not being paid, then you need to consider if it's worth the hassle.
Step 1
Set up a free email account with credentials you are willing to share, and importantly a password that is no relation to anything you use personally. You are potentially going to hand this account over to your former employer.
Step 2
Transfer the accounts to this email address. (This is the long tedious part)
Step 3
You have 2 options
Option 1
Reset the password on those accounts, to something not what you use personally, that you with your former employer
Option 2
Pass the details for your new email, purpose-built account over to your former employer, telling them to change the password for their protection. Then get them to use this account to go through the password reset procedure for each account.
Side note
Your former employers have created this mess themselves on several levels.
- Not having company email addresses which they gain ownership on your departure.
- Not having a company email address for this purpose (though this is pretty common)
- Telling you not to come in while there was still work to be done.