I did try with Captcha, verified registration method, unique email ,upgraded to the last stable DNN 7.2.2 version, and also not using custom registration page.

None of them worked.

Still it is creating users and sending user registration messages.

This is being a headache now.

Please can you give me a solution? How we can prevent this? Is there a way to this bot or something?

Please advise.

So nobody can register from that country? (ex: china) that could be only one user's work and not allowing all for that country is not a solution. Please give us a good solution as this is a clerical case for all DNN sites.

I feel your pain. I have exactly the same issue as you, and tried the same things.  I'm still searching for a solution without making it difficult for legitimate new Users of my site. I cannot turn off Registration because it is a subscription-based site seeking new sign-ups every day. 

Huge headache.  Keep us posted if you find anything, and I will do likewise.