Well, that really depends on the agreement. Yes, certainly some non-compete agreements are enforceable. Courts have repeatedly held that for an agreement to be enforceable though, it must be narrowly focused to protect a legitimate business interest.
So, an agreement that has literally no restrictions on it at all, either in geography, against specific clients or no time limit, certainly would not be enforceable. However, I've never seen a modern company attempt that sort of agreement, so I question what is actually in the agreement.
Certainly, an employer can stop current employees from competing for the duration of their employment, but once employment ends, there has to be a limited focus in the agreement in length and scope or no court would enforce it.
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