The back story: my forums are private and only certain people should have access to it's contents.

The problem: anyone can access the contents of my private forum through Google cache

Need a solution to stop Google from caching my closed private forums!

I already deactivated all the bots (including Google) but I doubt that will do the trick.

Thank you for your prompt help with this!

Hello,
If you have deactivated the Bots they won't log into your forum anymore to read data to put onto the search engines.
For extra measures you could remove their forum permissions.

1) Head over to your ACP (Advanced Admin Panel) and click the "Permissions" tab at the top,
2) Click "Groups' forum permissions' on the left hand side and select the "Bots" group from the drop-down box before clicking Submit,
3) Tick the "All forums" box before clicking Submit immediately below,
4) For each forum, select "No Access" from the drop-down box before clicking "Apply all permissions" at the very bottom.

As for what the search engines already have, unfortunately there is no way to remove these - you may get away with contacting each one and requesting that the search indexes are removed, but I don't think/ know if that will work.

If you have any more questions please don't hesitate to ask and we'll be happy to help