Hundreds of fake user registrations each day

Each day I get over 200 spam user registrations. I've tried both the graphical and mathematical CAPTCHA, but to no avail. As a result, my domain is now blacklisted everywhere resulting that my email end up in junk folders or aren't delivered at all because the system sends out hundreds of spam activation emails every day.

Is there anything I can do to prevent all these fake user registrations?

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Hundreds of fake user registrations each day - Brent, 2017-12-12, 07:44
- Hundreds of fake user registrations each day - Micha, 2017-12-12, 08:06
  - Hundreds of fake user registrations each day - Brent, 2017-12-12, 08:18
    - Hundreds of fake user registrations each day - Brent, 2017-12-12, 08:22
    - Hundreds of fake user registrations each day - Micha, 2017-12-12, 08:27
      - strongly recommended update, amendment - Auge, 2017-12-12, 08:55
        
        strongly recommended update, amendment - Micha, 2017-12-12, 09:00
        
        Update didn't change anything - Brent, 2017-12-13, 14:21
        
        Update didn't change anything - Auge, 2017-12-13, 15:20
        
        Update didn't change anything - Micha, 2017-12-13, 15:53
- Hundreds of fake user registrations each day - Auge, 2017-12-12, 08:49

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Hundreds of fake user registrations each day (General)