This is missing the point. The problem isn't that some designers are losing work to 99designs. The problem is that spec work devalues the entire design industry.
I hate to break it to you but you as a seller do not get much of a say on what value your product has in the marketplace. This is as true for design as it is for anything else.
The value of a product is exactly what buyers are willing to pay for it. Saying things such as "spec work devalues the entire design industry" suggests clearly that your definition of value is not in line with your customer's definition.
When this is the case, you basically have two options:
1. Demonstrate an increased value to the customer that you bring that they are willing to pay for, or;
2. Lower your price to compete.
Walling off your industry to preserve profits that aren't in line with the real value of the product might be desirable but it is unsustainable in the long run in today's modern world. Ask the music industry.
I upvoted for the rest of the comment, but this bit seemed off. Mainly because the crowd-sourcing idea relies on the respect of copyright even more so than the music industry.
iTunes is a perfect example that given the correct price point, people will pay for music. The big issue is that price point might not be high enough to support the massive marketing and promotion machine that exists. That isn't the consumer's problem though.
In the end, it's actually a perfect example of my point:
When technology reduces the barrier to entry to next to nothing, industries that were predicated on high barriers need to adapt, or they will die. It's a good rule of thumb that the moment you start to do something other than talking to the customer of your service in an effort to preserve revenue, you have a value disconnect with your market.
Basically out of all the people who love music, a large subset of them will pirate it without qualms, and that's all that's really going on here. The changes brought about by the internet have just shrunk the overall industry, rather than allow it to be reshaped by visionary businesspeople or artists. 99 Designs works because due to copyright protection designers feel confident about showing work before receiving payment, meanwhile the music industry is failing because people feel confident about avoiding the legal sanctions of downloading copyrighted material. That it should be like that is just an accident of law enforcement practices and tracibility.
Designers, especially web designers, are working in a highly saturated market. Even in the rural area where I live, I constantly meet designers who want to be in the running for the contracts my firm awards. The low barriers to entry and the seductiveness of the industry have created a situation where many folks are claiming to be designers.
Furthermore, designers are working in a field that requires a unique blend of talents. It's not enough just to be artistically talented and proficient with design tools. The key ability a designer must demonstrate is to be able to, via conversation, grasp the vision that a client has and translate that into a coherent design that provides a fluid user experience.
With so many parties competing for the same business that key ability is the deciding factor as to whether you'll get work or you won't.
If it's my firm and you're new to us, you're going to have to prove that you have the aforementioned key ability. And that means...steady yourself...spec work. Not much, mind you, and not more than is necessary, but enough to prove that you're not just another wannabe with an artistic bent and a Mac.
Don't worry, we're not getting free design work because if we're going use your work, you're getting the gig. If you don't get the gig, it's because your work doesn't cut it.
If that's too much to stomach, no problem. I'll grab the next designer's business card off of an ever-growing stack.