Blocking agressive visitors

In the past I (thought) I could block certain agressive visitors by adding their IP with ‘deny from’ to my access list. At a certain moment the IP’s changed to longer strings (retalted to V4 and V6?) and I got the impression that these continually changed without any grip on them anymore.
Are there ways to block such visitors (esp China, Russia, Ukraine) from visiting my sites?

The are probably bots, so you could use fail2ban to block them completely.