Please suggest me some way to kept me out of that trouble.
If you are using PunBB, or similar, then most of these products now support "CAPTCHA", and you can activate that option for new registrations.
And/or, you could change the registration code to ask the new user a very simple question - but one which an automated program would not understand. Such as: "What's the product of 2 and (2+1)". In the code, vary the values, operations, brackets, etc.
Frequently, new registrations have to be confirmed: the registration system sends an email to the new member, and expects it to be confirmed within, say, 24 hours. That should confuse some automated systems.
Sometimes, all new registrations have to be vetted by a human, but that might increase your own workload...