[bredren]

Multiple consecutive signups from the same IP address is one way.

But as GP said, this and other signals require coding abuse detection, and then you must find a way to enforce this.

This is not the kind of activity you want to be involved in while validating a product concept and working toward pmf.

[mahalol]

In NZ some ISPs use cg-nat meaning that you will have many people under one public IPv4 address. Going by this logic you could ban a whole ISP in one go.

[selcuka]

Also mobile networks have typically one public IP address.