I have notice a very simple method of blocking avatar's used by spamming agents and manual spammers.
Lets look at this manual spammers avatar
Each image has its source code I have attached the avatars image source code
It can be used as a defense against this spammer as the spammer uses the avatar time and again. If this code is blocked and because of the block the spammer can not register. This type of spam blocking would be a great thorn in the side of not only manual spammers but spamming agents.
As I have promised people here I would come up with new spam blocking ideas this is one of the best ones so far. I am rather hoping that a plugin developer will take this on as a project. Or even the Oxwall software engineers.