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A good idea is to change the signup slug to something else. This will help significantly. Also, if you don’t need to provide blog registrations, then turn this option off.

The problem with bundling a solution in the core is spammers will eventually get around this and it will become useless. The best way to fight spam is to have something unique on your site that stops them in their tracks. A completely unique signup slug is a good way of doing this.