I've not heard of this happening, have you experienced it?
I guess I would just advise to go with a trusted freelancer with a good reputation and maybe have a contract in place with non-disclosure and non-compete clauses.
Generally, I think freelancers are freelancers because they don't want the hassle of everything else you have to do to make a game successful other than actually make the game; e.g. marketing, promotion and distribution; so are happy to just be paid to make your game and move on to the next one.
Also, it's very rare that a game idea is so unique that it would be worth stealing, as most "new" ideas are usually just mix of established genres and mechanics.