Please tell me, is there some kind of ban system if there was registration using a username that does not exist?
We are providing such system as part of load balancer. Our implementation is not publicly available, but you can implement it yourself.
We use Kamailio as a proxy/load balancer. You can use modules like the Pike module to help block IPs if too many failed requests. pike Module
Handling SIP Flood Attacks Using Kamailio
Note that using Kamailio yourself is very DIY so not something you would implement lightly, might need professional assistance if you haven’t used it before.