If it's a problem with spammy blog comments I would recommend to just remove any kind of input on the site and ask people to send you an email with their questions and concerns.

Be sure to use a separate email and give it to readers on your about page via some language like "questions (dash) and (dash) comments (at) (this domain)".

If it's for account signups just send an email confirmation link and possibly include a code in the email that has to be submitted manually as well.