All websites, but especially Drupal sites with open registration, are subject to relentless attacks from malicious users and scripts. You can suppress a significant proportion of these with http:BL.
http:BL is a Drupal contributed module which interfaces with Project Honey Pot. Project Honey Pot seeks to identify the Internet addresses from which certain kinds of bad behavior emanate including email address harvesting, content spamming and dictionary attacks. It does that by enlisting websites that include specially crafted web pages in their sites that "normal" users never hit but the bad guys step on with their scripts. The payback for hosting the honey pot is that requests from these "bad" Internet addresses can be blocked.