What do you think the chances are that someone shares an IP that plays on a server that someone else in the same IP has banned in? VERY LOW.
It's far more beneficial to just have a ban button than it is to prevent someone from being banned for sharing an IP address.
And circumvent? Yes people could use VPN servers but eventually they run out of options and would have to pay even more for alternatives. It would be very costly to be banned that much, and doubt anyone would go for it.
The simplicity of just banning someone who constantly trolls a community is what we would expect as a server host in most games, and when it's as easy as it is to implement into this one there really is no true excuse for not having one.
Lastly, what is to stop a troll when they know nothing is stopping them from coming right back and using deception of another name to gain trust and repeat the same process?
We shouldn't have to use tools like Wireshark and Firewall tools to manually ban people and frankly it's ridiculous that we do.