Enforcing non-compete clauses

That would depend on the jurisdiction you're in and the wording of the non-compete clause (a lawyer's answer, I know). Some places do not allow non-compete clauses in employment contracts. If they do allow a non-compete clause, most courts will require that it be very narrowly drafted, that is very limited in the type of competition, territory, and time-span. Courts tend to frown on non-competes that effectively keep the ex-employee from working.