If you allow public registration for commenting and/or blogging, it won't be long before you discover spammers abusing your system.
Tools like Mollom help a lot, but they won't catch everything. You need to check your registrations at least every morning and clean up the mess. If you let it get out of control, your site will become a favorite for SEO spammers, many of whom are paid a piece rate for posting links on websites.
We use a heavily modified version of the Abuse module. (The released version is badly broken and largely abandoned.) Our version includes a user operation that lets you unpublish all the "contributions" of a spammer from a user list page (admin/user/user). This ensures we don't miss anything tucked into odd corners of the website.
Since our changed version hasn't yet been released, and because some people may prefer to use other tools for flagging abusive posts, I created a stand-alone module that implements a bulk operation to clean up after a spammer or other abusive user.
The Ban Unpublish module works nicely with the standard user account listing, with the Advanced User module, and with any views you create with Views Bulk Operations.
The development version adds bulk removal of bans and blocks, as well as bulk deletions.