“Unfortunately it also stops the occasional human visitor leaving a comment but I think it’s worth it.”

What no bold font for this? It’s pretty serious: in trying to lock out spammers, you are locking out humans too. At the very least, you should mention in your comments form that cookies are required, else merely having high security settings will block someone.

Stop helping spammers to break the web and find a better way to do this.

I’ll probably unsub from your site soon because it’s too fiddly to interact with it.