I had to use iptables to block a spammer from my Linode.

Some spammers keep changing IP addresses.

Is there a "recommended block list" I can use? Is there a way I can set it to automatically update when spammers change IP address?

Perhaps, it kind of depends on what the offending behavior is. Is this forum spam? E-mail spam? Webform spam? None of the above?

mail spam: a decent RBL will take a decent chunk of out your spam. It won't block all of it, but it's easy to set up and requires very little resources

forum spam: get a better captcha or user signup verification process

webform spam: get a better captcha