How to block spammers in phpBB3?

I have a phpBB3 site that has been spammed once or twice. The user will post gibbersh into a few of the forums... I notice, go in and delete it, no big deal.

What is keeping us clean for the most part is - the site requires users to signup, and confirm their email, before being allowed to post. That deters bots, but this appears to be a real human going through the motions of activating their account, navigating to a specific topic, and posting their useless annoying message.

I do not want my real users to be asked to do anything additional.

Solution

Spam prevention is a multi-level process. The MODs and settings I have listed below work for me, but don't stop all spam by any means. As you mentioned, it appears that a person is behind the scenes registering some accounts. This is true. Later, that person returns or has an automated bot use his credentials to post something spammy. In those instances, your only option to to react. You won't be able to stop everything.

The first step, as was mentioned in the comments, is to set up an approval queue for the first post. In PHPBB 3.0.3, this option was added.

Browse to your Admin Control Panel
Select the General Tab
On the left hand side, under Board Configuration, select Post Settings
Find the Enable Queued Posts option and set this to 1

You (or your moderators) will have to approve these posts for them to appear. Once that first post is approved, the user will be able to post normally.

The next option involves modifications to the board itself. One such modification is the Anti-Spam ACP. On it's release page, the following features are listed:

Akismet Integration
One Click Ban
Stop Forum Spam Integration
Spam word catcher (like word censor, except makes posts by authors with less than X posts need approval if the post contains any of the words in the list)
Disabling of certain profile fields until X number of posts (or require or always disable)
User Flag (Allows you to flag a user (and unflag after they've been flagged). Once flagged, actions performed by that user to their profile, or editing of posts, or adding posts gets listed on a log in the ACP.)
Spam Log
IP Search

It also lists that it "it has the power to completely stop phpBB3 spam." Take that with a huge block of salt. I've been using it on my board for a few years. It definitely reduced the amount of spam I was receiving on a daily basis, but it does not stop it completely.

A third option is the Advanced Block MOD. This adds multiple blacklists to your forum. It allows you to add certain weights to specific blacklists (you pick what you like best) but does have a decent set of defaults that I'd suggest you run with for a little while. It adds a Block Log to your ACP as well. You'll quickly see that there are many bots being blocked with this mod. I noticed while writing this that there is a new version coming out soon, but hasn't made it through PHPBB's validation process. More information, including screenshots of those features are here.

Using a combination (or all 3) will significantly lower the rate that spam appears on your board. As I said earlier, it won't eliminate it, but it will make it more manageable. Your existing users won't have to do anything additional with these 3 changes. New users will have 1 additional step - they will have to wait for their first post to be approved. After installation and configuration, the administrator won't have to do anything either, other than monitor the logs to see if any configuration options need to be modified.