I literally just posted an answer to a question similar to this on another thread so I'll post it in here as well:
Tough situation to be in, but your gonna need to get VERY creative here. Reputation management is tough, especially in your case, considering the bad review is on a .GOV.
Plain and simple you will not get anywhere building content to your own website. Google generally gives 1 SERP listing, and potentially 2 or 3 depending on if the internal pages are worth while enough. Adding more content to your own site isn't the answer.
You will need a budget here and a good one at that to push down a .GOV. Find popular pay per post services, these are services in which you can get a post on a popular blog/domain. There are also a ton of bloggers that offer 'sponsored posts' on extremely popular blogs that have incredible PR. You can find some for $200 - $700 ++ PER POST. Now whats important is to have KILLER content written about your site, with your business name as the post title and maybe even something catchy that will attract click throughs from the search engines. Click throughs are huge in gaining top spots in the search engine.
After getting roughly 10 or so of these, you'll need to perform an extensive paid link campaign and link building campaign to those blog posts that you just paid for. Don't even hold back here, if your business is truly worth it then cough up the money for a solid link building campaign for these blog posts that you just got. Go out and find some private link brokers that have incredible links such as PR6,7,8,9 homepage links. You can get some for around $200-300/mo. Buy one for each post that you just made, and a few if you can cough up the expense for a little while. Yes, this will be expensive but this method will work.
I'm speaking from experience here, I ran a $50,000/mo. reputation management campaign for a HUGE company that ended up getting shut down by the government. I won't name the company, but it turns out they were a huge scam. At the time, no one really knew it. We took the front page and cleaned it up using the methods I described in this post.