How can I prevent that banned users/IP can still login at our forum? I've banned several IP's but they keep comming back browsing & searching our forum...
I do believe you when you say it will cause a high load on the server, but actually I dont understand why. Upon registration, you only need to check if the IP is on the banlist or not... so where would the high load come from ?
You'd have to check the IP against banned IPs on every single page load to keep them from viewing anything anonymously.
You didn't mention registration before - you said anonymous browsing and searching.
You can block an IP address with a .htaccess file. Many web hosting control panels have a "IP Deny" feature. I know Cpanel does. Just type in the IP, and the IP is blocked from the server.