Can you not access the admin area? I am not saying that this s defiantly your issue. I could still access the admin area when I had the spam issue.
If you cant, then it sounds like it may be a bigger problem. How did this error start? Was it after you added some new code? If so, it could be something stuck in an infinate loop.
If it is comment spam, and you can't access the admin, you will probably have to access the database directly (Something like PHPMYADMIN) and check the PageComment table. You will be able to see from there if there are a lot of spam comments. If there is a lot of spam, you will also need to delete the comments through this system.
Have you tried doing a /dev/build/?flush=all Maybe clearing your template cache would help.
Have you checked your servers error logs? They may be able to shed a bit more light on the problem.
You can also add basic spam protection to comments by using adding:
To your _config.php file. I have found this simple field has cut pretty much all out spam comments out.
Finally, what version of SIlverstripe are you using? Have you got any modules installed? Are you running this on your own server, or a shared host?
Hope that's of some help.