how to block spam optins from certain emails

I have a couple different forms on my site from my Aweber account. I am noticing that I am getting quite a few optins who don't double optin. In other words, they are filling out the form (or a robot is filling it out), but they never confirm the subscription.

I can block future subscriptions from these emails in my Aweber account, but is there a good way to block the bots right in the beginning using a plugin? I was thinking of Wordfence or is there something available through WPMU Dev that does not require extra fields like the Captcha?

I want to keep my optin in form very streamlined with just one or two fields to fill out, as shown on my home page: http://wordsandwriters.com.

Thanks, Joe.