It's not that simple. Some jurisdictions make such contractual clauses null and void on their face unless "work for hire" conditions prevail -- and "work for hire" generally (although not universally) means tasks carried out in the normal course of employment. A contractor hired for a specific purpose does not normally meet the definition of "employee" for the purposes of "work for hire" law. The situation is further complicated by the fact that most online relationships of this sort involve at least two jurisdictions, so while you may be able to prevent actions by your contractor in, say, Nebraska, that wouldn't prevent him from operating in California or Romania. Even if you can get a judgement against the contractor in Jurisdiction A, that doesn't prevent him from operating in Jurisdiction B, and laws concerning what legally constitutes "operating in" a jurisdiction may keep you from enforcing the judgement in Jurisdiction A as well.
Again, the idea that your idea is valuable in and of itself is something to rid yourself of entirely. If you can execute, execute well, gain traction early, and remain a moving target by incrementally improving what you do, you have a winner -- and that's going to mean letting your technical person/people in on the game. If they have a vested interest in your success, they're not going to compete with you. If you want to do a "once and for all" job with hired labour and your idea is anything like good, you'll have your doors blown off quickly by people who've seen what you've done and can not merely copy it, but do it better.
It doesn't take much to copy most things -- any idiot can build a Facebook clone, and many idiots have. They didn't need any insider information. Persuading a half a billion people to move from Facebook to their app, though, is the hard part -- and the only reason that so many people are on Facebook is that there are so many people on Facebook. Facebook has always been that way -- everybody you knew was on Facebook. That's because they started with a small, closed community where everybody could know everybody through only a couple of degrees of separation (and you didn't have to be a user to have a profile at the beginning -- other people could make one for you). By the time everybody knew about Facebook, the community was large enough that migrating people off of it became the major problem.
The stuff that is hard to copy isn't a vague idea. Google got big because MapReduce and PageRank gave them a fundamentally better way to search the web. Monetizing that through advertising was an equally hard computing problem. An "idea guy" wouldn't have been able to pull it off at all, no matter how many CRUD-churning code monkeys he hired -- it took a pair like Sergey Brin and Larry Page to make it work. There were already a lot of big players in the search space, but the searches were so-so at best and the ads were almost always completely irrelevant. Getting the computing problem right made the difference (and still does).
The chances are pretty good that whatever idea you have, it's a lot closer to the Facebook end of the scale than it is to the Google end. If it's simple enough that you can hire somebody to build it to your specifications, it's simple enough for somebody else to copy at a glance, so don't worry about the guy you hire stealing your idea -- anybody else can do the same thing. If you want to win the game, then play, and don't stop playing. Build what you envision, get customers, listen to them and make it what they want/need it to be. That takes time, not a one-shot contracting deal. Give people a reason to join, and keep giving them a reason to stay. If you can't do that, then you've already lost.