Well it depends what you are hiring a freelancer to do really, but a great way to protect your ideas is to only give as much information as they need to complete their task. Beyond that you should look for freelancers that only specialize in a single area. For example:
You have found an untapped keyword that people are searching for like crazy for a specific weight loss product. You want to build a website, advertise this specific product, and do a mass blast of profile links.
Things to protect: Keyword, Product, Domain Name.
Go to your favorite freelancer board and hire someoen to build the website, when interviewing them ask them if they have any seo skills as well (beyond standard onsite seo) if they do then look for someone else. Once you find someone who only knows how to code websites have them build the site but allow you to enter the content yourself later. Also do not give them the name of the domain either, have them do a generic logo.
Next hire someone to do the logo, do the same type of process when finding a graphic designer. Ask them if they can also build websites. If they says yes, or say they have a friend who can, then dump them and move on to someone who only knows how to run photoshop.
After that you can publish your website, add your content and product link and go back to the freelancer board to find someone to build profile links. Do the same filtering and find someone who doesn't know how to build websites. Now instead of building links directly to your money site, build some web2.0 properties and point all the profile links to them. Once the task is completed then you can link the 2.0 properties to your money site.
In the example above none of the freelancers have enough knowledge or skill to complete the project themselves and steal your idea. I know this will not work in all instances such as things like craigslist posting where you will have to provide your ad copy to the posters, in those types of instances look to outsource the building of a bot to run yourself.
Really its not hard to find these types of people, I taught my article writer (who knew nothing except how to research and write articles) how to create profile links in a matter of 20 min. He still has no clue why he is posting all these profiles or how it converts into money or how I am paid from it. He is just happy to be paid for doing it.
Another note; If you do decide to break up a project between multiple freelancers, be sure not to break up any coding tasks. From experience I can tell you that mixing coders in each others code ends poorly 99% of the time.
If you find a good freelancer stick with them. Make them happy and keep telling them there will be more work coming. Give them a bonus for quality or fast work.