Based on forum topics like:
How Can We Improve Our Body Fitness?
I think spam is an issue here. Of course it is everywhere.
I would like to suggest people consider using ZBBlock as general protection with any php script including Oxwall. (See http://www.spambotsecurity.com/zbblock.php ) It's free, the developer is helpful, there is a forum. One of the first things I did after setting up my oxwall was insert the 1 line of code required to run the script. (I insert in the configuration file. For me, it was a 1 line of code addition because I already use ZBBlock on other sites.)
The script drastically reduced visits by 'bots in general, and so helps any spam filter work. It's not a plugin for oxwall because to work best, it needs to trigger before anything else loads.
Some people like using ZBBlock others don't. But anyone who is having spam problems or issues with 'bot attacks should try it.