(hopefully) brilliant idea to prevent spam
 

I just had this idea. Let's say someone would create a website that allows board owners to post usernames and IP addresses of people who are known to post spam. In that case, all other board owners could take measures to prevent these people from registering. Like banning the posted IP addresses in advance. Would that be a good idea, or does something like that exist already?

Great idea. it would be a site i use

This wouldn't work because we live in a world with dynamic IP Addresses and proxy programs.
Another point is What's stopping administrator from adding, say, your IP Address to the list?

Even if we all had static IP's, proxies didn't exist, and all administrators are honest they would also have to be not lazy. I am not going to another site to fill in an IP Address of a spammer.

<a href="https://www.stopforumspam.com/spamdomainsandips" target="_blank">http://www.stopforumspam.com/spamdomainsandips</a>

There is also botscout.com