#1 ALWAYS APPROVE A REPLY
Currently in v1.8 or later if you go to Settings > bbPress Moderation
You will see the settings menu
Display pending posts on forums  Always display
E-mail me Whenever  A topic or reply is held for moderation
Do not moderate  A topic or reply by a previously approved author
Anonymous topics and replies  Always moderate
For registered users if you select 'Do Not Moderate', any previously approved users will not be moderated.
For anonymous users 'Always Moderate' should be left checked. As turning this OFF effectively turns off the moderation within the plugin.
CURRENTLY, bbPress does not log email addresses of anonymous users, which means there is no way to identify if a given anonymous user has posted before.
Technically, the bbPress plugin sets anonymous users to post_author = 0 [admin is 1 and other users follow in creation order]. bbPress does not record user email for anonymous users when it sets post_author = 0 so there is no way I can tell from the data bbPress collects.