Can I stop an employee going to work for a competitor?

You can include post-termination restrictions or “restrictive covenants” in the contract of employment.  However, these are designed to protect legitimate business interests and should not go beyond what is reasonably necessary to protect those interests.  They are not allowed to prevent or restrict competition in general.  This means that such clauses need to be carefully drafted and are likely to restrict the ex employee only from contacting or soliciting business from customers that he has personally had contact with.  They also need to be appropriate to the level of the employee  – more senior employees are likely to have access to more business critical information, which would justify more onerous restrictions.

Restrictive covenants that are deemed to be too wide or inappropriate for the person’s role are likely to be unenforceable.