That can actually happen to any forum software, even the home made ones. Heck I could do it to yours if I wanted to right now. The problem of verifying if the person is real or a bot is extremly hard to fix, plus any person who has the script real or not can spam boards with one click. Not only that, but once you do fix it they rewrite the way the bot works to work around the new measures. As examples, one fix used to be to put put an image with letters on the registration page since a bot couldn't read it. Now these programmers make smarter bots capable of reading those images. Even further another way of preventing it used to be to require a valid email address to register and login and post. That stopped them for a while, but now the bots are actually able to enter an email address, receive the activation email, then use the code in that email to login to spam the board. No matter what measures you use eventually someone will get around it. Then you fix it, then they find a way around your fix. It's a never ending loop. Unfortunately the only true fix to this problem would be to stop posting alltogether... but that defeats the purpose of having forums.It's still got some issues that need fixing, look through the results of http://www.google.com/search?safe=off&q ... n+Ford+smf
Now you don't have to take my word for it, but you will see it happen eventually.
But I have developed forum software for years and know what I am talking about. I have been part of alot of forum projects, both major, home made, and helping fix bugs for other projects. As example I had a major part in the forum software that runs this board here, YaBBSE. I made 2 homemade forum softwares, 1 of which I still use for my website. I have had ties to other forum software like mercuryboard, azbb, yabb, and smf for coding, themeing, bug fixing, and support as well. Plus I have tried tons of other software and wrote converters for them to move them to my home made ones.
But anway, this same thing could happen to yours, and it can happen regardless of whether the source is available or not. Heck it's even happened to my homebrewn software even when the source wasn't available. Yes I fixed it... but they worked around it and I wrote another fix, basically and endless cycle. All it takes to do so is examine your output to the browser and submit post and get data to your board.
Oh and you might want to get in contact with me in private via instant messaging to talk about some vulnerabilities I have found with your software. I am able to hijack accounts, learn users passwords, and cause the software to automatically log you out of the forums, and I could also spam your board just like what you saw happen with the smf forums. Lucky for you I am not a malicous person or a spammer.
Please remember, obscurity does not always make you safe. Just because your software is homebrewn doesn't make it more secure than others. I have no access to your source code at all, and was able to find these types of vulnerabilites.