Hello,
No this locking feature can not be used for forum users, this is available for the users logged in the administration interface - CMS Desk or Site Manager.
What you can do is to limit the session timeout via web.config file e.g.:
To limit session for 20 minutes:
<authentication mode="Forms">
<forms loginUrl="CMSPages/logon.aspx" defaultUrl="Default.aspx" name=".ASPXFORMSAUTH" timeout="20" slidingExpiration="true"/>
</authentication>
Best regards,
Martin Danko