Hi Frank,
The next best thing is to try blocking the spammer's IP address from accessing the site.
1 - First you need to find out what his IP address is. For that you will need to track the site's visitors more closely for a bit. There are numerous ways of tracking that, but here is a module that will show you a full report of access to your site, including their IP address: Advanced Visitor Analyser - I have not used this module before, but it has a trial available.
2 - Then once you know the IP address of the spammer, you can watch this video on How to restrict access to DNN based on IP address
One thing to notice is that the spammers might be using various IPs, so you may need to filter out based on an IP range. That is shown on the video as well.
I hope this will help you.
Cheers,
Aderson