Help with old accounts, getting hacked

[dnikola]

Hello guys,

i would to get some kind of help, more advice what should we do in situation (or you will if you were in our situation)

- Few years ago our forum get hacked (that was situation when a lot of VB forums get hacked)
- All users information's like usernames / passwords / email's were published on some forums
- Some of our forum users (spammers) get that information and grab that list before it was down
- In main time we have done reset of all user passwords, but problem is that a lot of users had same password on forum and email, so this guys can access emails and reset users passwords

This is what we suspect that is ongoing situation, because each day we have account which last post was more than 5-6 + years ago and than account start to troll and make spam on some political hot topics and etc.

What should we do, if you have some suggestion?
Do we miss something in order to "protect" this old account?
Is there any tool to monitor a detailed account changes with IP's or etc?
Should we just deactivate account if they don't have post's more than x years?

Thanks in advance
Nikola