When a web host uses account automation software for user sign-up how can they control security in terms of keeping fraudulent users and spammers, etc. from automatically signing up and putting their evil content on a server?

web hosts still have to face this problem because it is difficult to say Whether it’s automatic or manual.

I think it’s a matter of human monitoring.

Firewalls and automated servers monitoring is a good way to prevent spamming eg. CPU load, Mem Load, database loops, mail queue etc.

What I know is that these spywares etc needs to access a few dlls (windows) or need right permissions to execute. You can use a script to prevent client applications to use these files.

I think you’re right because no matter how good the systems, you must have a human monitoring on all your server