Welcome to the forum!

Most users have dynamic IPs that change every few days. They can even register from other computer from work, school or friend for example

Anyway, there's no option to block users by IP in b2evo. I can give you some tips if you want to mess with PHP code.

The host of my website allows me to block ip adresses through the ht acess editor but it ask me for the directory to select , but i don't know which one. Any ideas?

Try this

domain.tld/path_to_b2evo/htsrv/register.php

Thanks for your help but what should i do with the code you sent me, i'm thinking its easier to create a ip blocking file and place it in my ht acess editor, i have a code but i don't know how to create and place the file in my htacess .

It's note a code, it's the URL you may want to limit access to. Try this in .htaccess

<Files register.php>
    order allow,deny
    allow from all
    deny from 12.45.67.90
</Files>