The system is weighted against negative votes as it is. The system is designed to be more of a 'body of work' kind of thing. Isolated incidents of votes in either direction shouldn't have significant impact.
Yours is a good example. It took you a lot more votes to get a negative rating than it took positive votes to get you out of it.
One change I am considering on the system is further restricting it's use to more established members. In other words - raising the post count required to leave reputation scores.
Still evaluating things. So far it hasn't been much of a problem - for most people.
In your case, you got a few negative rep votes over your spammy post:
"One short form gets you up to 5 quotes from leading designers. No obligation to buy. Don’t waste time looking for designers and posting on message boards; let them come to you! Get the best price and design firm possible!
So you spammed the board, got some negative feedback to let you know it wasn't appropriate and since then have recovered back to a 0 very easily -- all on an account with just 50 or so posts.
That tells me things are working just fine.