The Anti Spam by CleanTalk module might be another anti-spam module that could fit in your case. Some more details from its project page:
Invisible antispam without CAPTCHA, questions, puzzles, counting animals, math and etc. Just install and forget.
CleanTalk is a SaaS spam protection service for Web-sites.
CleanTalk uses protection methods which are invisible for site visitors. Using CleanTalk eliminates needs in CAPTCHA, questions and answers, and other methods of protection, complicating the exchange of information on the site.
Optionally you may also want to look at the Comment Trust module. Here are some details about it (from its project page):
... auto-approves comments by users with previously-approved comments. It rewards trusted commenters with immediate approvals, while requiring you to approve the comments by new visitors. Works for both anonymous and authenticated users.
You can configure the threshold of approved comments an email address has to first achieve before gaining auto-approval. You can also provide a list of email addresses (or patterns of email addresses) to always exclude from auto-approval.