Hi there,
I would love to help you with blocking spam, although your plan is good as well , but google re-captcha option works the best , as its backed by tons of algorithmic actions that blocks spam bots and allows only genuine users. I can help you with implementation of any of them. I am 100 % confident that I can do this. Will start immediately and will get the work done as per your specific requirements.
Looking forward to the opportunity to work with you and establishing a long-term working relationship.
Lets discuss this more over chat please.
Thanks,
~ Syed