BuddyPress.org

The limitation of WP-Ban is that it’s not working at .htaccess level, so it only really does it’s thing if a spammer is polite enough to access your site normally. You might want to look at something like a plugin that’s going to ban IPs and referrers at the .htaccess level.

Also, had a quick look at your site – I presume you’re talking about http://young-people.ch ? I notice your register page is still /register (albeit translated) – have you tried changing this to something else? There’s eevery chance that the mere translation of the standard ‘register’ slug won’t slow the spammers down.