As long as there have been stock markets, there have been suckers. And as long as there have been suckers, there have been charlatans to take advantage of them. In the famous "Mississippi Scheme" of the early 1700s, an old-school stock promotion nearly bankrupted much of Europe. Not long after, scam companies in England's "South Seas Bubble" took down the entire nation and economy.

Charlatans continue to plague the financial markets, and I'm not (just) talking about outfits like Goldman Sachs (NYSE: GS) and its "slegal" practice of not fully disclosing just who might be putting together a synthetic mortgage-backed security in order to short some of the worst loans out there.

I'm not talking about other government-supported parasites, either. Companies as diverse as derivatives disaster AIG (NYSE: AIG) or a billionaire's battery-car plaything, Tesla Motors (Nasdaq: TSLA), may be, for lack of a more polite description, full of ... it. And yes, they may be suckling at the taxpayer teat via undeserved loans or other support. But at least they have some kind of business.

That's not the case for the good (by which I mean awful) old-fashioned penny stocks, which are promoted all day, every day, by the stock market's equivalent of the flea-bitten carny.

You know who I'm talking about: the bozos who spam your email with faux stock tips, touting tiny, mostly revenue-less wonders as the next big thing. These days, the come-ons often appear in the guise of market news, with a couple of legitimate tickers thrown in to try to lend respectability to what is otherwise a good old-fashioned scam.

Other common templates for bogusness include wink-wink name-dropping, such as a recent slug of supposed Apple (Nasdaq: AAPL) App Store beneficiaries trading on the Pink Sheets or the bulletin board. You're supposed to believe that some of Apple's success will trickle down to them, simply because they've changed their business description from "Nevada uranium miner" to "provider of iPhone applications."

To help illuminate this pervasive scam, eight months ago, I started a Motley Fool Caps account called TMFStockSpam. It aims to put the promoters' claims to the test. Our process was simple. Watch our inbox, but also give an email address to some of the spammers out there, and enter the junk they sent our way into CAPS -- with a big, red, thumbs-down. (We're also open to suggestions from the community. Simply post them in TMFStockSpam's blog.) The results remain online for the world to see.

So far, the evidence is overwhelming: Stock promotions will absolutely murder your portfolio. More than 90% of the promoted stocks rated by TMFStockSpam have underperformed the S&P 500, by an average margin of nearly 50 percentage points. If you'd put your money on these losers, you'd be half broke by now, with "totally broke" just around the bend. That makes TMFStockSpam one of the most highly rated CAPS accounts in Fooldom, in the top 100 out of more than 70,000.

If you want to review the claims and the consequences of stock promotions, TMFStockSpam's got more than 100 prime examples. Take a look -- and watch your wallet.

Seth Jayson had no position in any company mentioned at the time of publication. You can view his stock holdings here. He is co-advisor of Motley Fool Hidden Gems, which provides new small-cap ideas every month, backed by a real-money portfolio. Try any of our Foolish newsletter services free for 30 days. The Motley Fool has a disclosure policy.