In Need of Urgent Advice from Webmasters - Spambot Issue

Hello, I'm a website owner, and have a situation with my web engineer/host. My company hired him to redesign our existing website and build a new CMS. He also convinced us to use him as a host, and we moved our site to his server prior to our re-launch. Since the redesign we haven't needed his services (except once, when his server crashed), and simply pay for web hosting. Two days ago, spambots began attacking our site, posting an average of three to four spam posts a minute. We have been trying to delete the posts manually, but it's become unmanageable. We decided the best thing to do would be to have a captcha built into the site (unless our engineer had a better solution). My web manager contacted our engineer this morning, and he said he could install a captcha. He said it was an eight-hour job and he could get to it "next week." We are literally under attack, and have two people taking turns working around the clock to kill this spam, and cannot wait a week. I contacted a friend in the web community, and she had someone who could do the work right away. I called our engineer (only my second time dealing with him) and requested the site log-in information, and he said he was too busy to talk to me, and that I needed to submit my request to him via email. That was at 2 p.m. today; it's 11:30 p.m. and he has not replied. My company owns the domain, and the site, and we do not have any kind of maintenance contract with our engineer/host. I have idea what to do now. I do not have the manpower to have people kill each spam post manually, and literally have someone waiting to help me, but my engineer has me blocked. I want to deal with this issue quickly and professionally. What are my rights as a customer? How do I gain access to my website to get this problem resolved? Thank you so much!