Hi!
One thing that has worked for us in the past (other than Google reCAPTCHA) was to ask a simple question in the last form field that has to equal an exact letter, phrase or number (i.e. "What is 5 + 5?" or "What is the first letter of our company name, John Lynch Builder, LLC?"). This idea seemed a little silly, so we added an explanation "Please answer the following question to help us prevent spam from getting sent to our inbox:"