Delete or Do not delete their member account..
Simply just mark their email or IP as a spammer using antis pammer tool

Quote from Raden It is still not clear if they were robots spam or human spam in your site? ...

Especially for Human Spam...you have to determine what kind of users you would think of as a spammer in accordance with the terms of your site service
NOTE
Human Spam can be found almost on a crowded site traffic and receives user registration

As you can see from today's spammer list:

xia97802@163.com

eyedefinalmi@gmail.com

minnielrqx@hotmail.com

damian_malinik@o2.plma.

ssawuduiddri.su@gmail.com

rosiecota.1966@4freemail.org

22313moo@loonycoupon.com

porfirio_ochoa@exzurueck-gewinnen.comdarekmmorek@aol.com

natetardy2762@mailnesia.com

aaaaaasaei@loonycoupon.com

Many are the same address as the previous list, timed from a script to spam the site at exactly the same time everyday

163.com is from China

This is a direct target because of Oxwal 

Whatever is identifying all the sites as Oxwal sites, needs to be removed because it is quite clear the spammers script can find these Oxwal sites and spam the hell out of them

anti_spammer is passing these right through as well so they much have found away around 

have u tried this:

Quote from Raden Simply just mark their email or IP as a spammer using antis pammer tool

Cit 27, who wants to do this 50 times per day?

No one

I agree. I also have spambots and even with the plugin its giving you daily work to do.
Is this because of the powered by link? 

Quote from Bradford Page who wants to do this 50 times per day?

No one

if they are robot from the same Ip i think they can`t sign up again after you mark one of their email/ip using the anti spammer..

Please make sure if they are not a robot but only the bad user who register in your site...so you have to delete them for violating the terms of your site service..
I still not get users who use the all email addresses above

no relationship with the powered by link?

This site (oxwall.org: I think it's more popular than on your website and also using the powered by link) ->by only using watchdog and the simple sign up question but it is still safe untill now?