As Dana pointed out, you should probably think about signing some sort of agreement of confidentiality with the freelancers.
If you are still worried about the freelancers hurting your idea, try separating the work into multiple segments and provide it to multiple freelancers. Of course, this will take some managing, and you may need to set same ground rules for the coding, but this will allow you to provide the information to them on 'per need' basis. Harder, but works.
The last option I can give you, is try to do the v1 yourself. If you lack some expertise, hire a freelancer to do some work for you, but code the crucial parts yourself. This is what I would do.