For some time, there are spam registrations in DNN portals with which many website owners have to face. Now there is a solution of Interactive Web, which the company free of charge available to the community.
Interactive Web has built into a modified version of the registration module, the reCAPTCHA technology from Google. However, this makes the integration a little more complex, as it requires registering with Google for any website. However, it is worth the effort, as this spamming is prohibited.
The download for free from there after Registration