http://en.wikipedia.org/wiki/Restraint_of_tradeRestraining workers
See also: Non-compete clause
Restraining clauses in employment contracts are enforceable if:
There is a legitimate interest which needs to be protected. Examples of such interests include business connections and business secrets.
The restraint is reasonable, i.e. sufficiently protects the interest and goes no further.
Generally, if a restraining clause is found to be unreasonable, then it will be void. In certain circumstances though the court may uphold it either by construing ambiguities or by severance. Severance consists of the application of what is known as the "blue pencil test"; if individual words which make the clause excessively wide are able to be crossed out and the clause still makes grammatical sense, without altering the nature of the obligations, then the courts may be willing to sever the illegal aspects of the clause and enforce the remainder.
how could you stop some one taking your custy if they wanted to go with them, as you are technically stopping them from earning a living in there chosen field of work