You are able to disconnect your steam account once linked, as I would assume likewise for your other accounts you may link to the forums. As it happens though, if something is online, it's likely to be searchable via Google in any case, not much can be done about that.
Click your icon, and then 'Connected Accounts'. From there you can see a small list of social media outlets that you can link your forum account to. Once linked, you still have the option to disassociate said account from your forum account.
Please see attatched files for a visual, I hope this helps you.