I thought I would update people on how I am getting on. My hope was to post back to say I have eliminated the spammers but that simply has not happened. I have installed re-catcha and done the web.config modifications and none eliminated the spam completely. The re-captcha has made matters allot better but I have noticed that not all the IP's used to target DNN are on blacklists and I think this is why some get through. The re-captcha relies on upping the defence for suspect IP's and lowering it for genuine users. However if the IP is unknown the spammer can still get in. 

My current line of attack is to have an IIS IP blocking session about twice a week. I look in the windows event logs for failed URL requests and block these IP addresses. It seems to have some success for about 3 days until the spammer starts using new IP addresses.

It would be so helpful if DNN logged the IP addresses used when registering and provided some way to then click a button and block the IP address. I also think we need a DNN specific blacklist to target known spammer IP addresses. A honeypot scheme where by we all host dummy sites that draw in the spammers and then block those IP addresses.