If you allow any kind of user generated content on your Drupal site including comments, you will invariably start to see users crop up with user names like: "gyigrrgq", "porn", and "buy viagra". At best, the users sit in the user's table and do nothing. At worst, the users probe and then blast your Drupal instance with more spam than you can shake a stick at. Often the technique will be to leave comments on old content in the hopes that site managers won't notice it but it will generate some clickthroughs to whatever site the spammer is targeting.
What I like to do is set up a block of recent comments. Sometimes it is public and sometimes it is private to a single role depending the on the site-owner's needs. This is a very simple thing to do and will allow you to see new spam comments at a single glance. If you check your site every few days, you'll catch any spammy ugliness.
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