I didn't try to redirect this thread because I assume you're talking about real people sending spam. I only had the need to suggest the add-on. As with normal spammers, there is nothing you can really do unless you can somehow get their IP address. Then you could see if they had a suitable account and block their IP address. If emails are sent to your administrator account and not to an external provider, it may be possible to get the IP address. Although you should have a look around.
@Lammchen I wanted to recommend the OzzModz add-on for the Spaminator if you don't already have it. Although it is practically impossible to prevent anyone from using the contact form, in just a week of installing it it has prevented well over 100 bots from logging into my website. Not a single bot has signed up since I installed it, and I was hit hard before I had the add-on. There are loads of 5 star ratings. The add-on costs $ 24, but it's worth it in my opinion. Now I can focus on legitimate registrations instead of worrying about bots logging into my website.