First, report the junk faxes to the authorities (see http://www.junkfaxes.org/ for info). All faxes are required to have an accurate return phone number, and the sender is liable for some hefty fines.
Also, report the information to the SEC. They’ll also investigate and go after the senders.
It’s not usually the company sending the faxes. Most likely, it’s some shady investors working on a “pump and dump” scheme. (I’ll bet the prices of the stocks being touted are usually less than $5; these are known in the stock brokerage trade as “penny stocks” and they are not recommended except for experienced investors.*) What they do is buy a lot of shares of some worthless company, then talk up the stock in order to drive up the price. Then they sell and the stock returns to its normal level – which is below what those they solicit pay for it. This works because cheap stocks are very volitile because only a small number of shares are traded.
So the pump and dumper buys 10000 shares of Amalgamated Crapola at $2, then sends out the glowing reports. Others buy in and the price goes to $4. The dumper sells his stock at $4 (maybe even to some of the people taken in by the scam); then the price drops to $1. Scammer gets a big profit; everyone else gets burned.
*SEC rules prohibit a broker from suggesting a penny stock to clients unless they have a track record of buying them. Further, if you decide yourself to buy one, there is all sorts of extra paperwork warning you that you’re going out on a very big limb and brokers are supposed to actively discourage investors from getting involved.