Should be reciprocal if it exists. I.e. if the employee leaves, the company would be barred from hiring anyone else for any remotely similar tasks. Make them taste their own medicine and decide if it's worth it.
Only do unto others that which you wish others to do unto you (or something like that, you get the idea).
It's common for executive's lawyers to negotiate contracts with the firm before accepting an offer (including what the firm can or cannot do to the executive without penalties). I wish it were more normalized for ICs.