Is the clause that prevents an employee from dealing with the company's clients for years after leaving their job permissible in Islamic law, and is it permissible for them to deal with a client despite this clause?

The condition imposed by the company on its employees not to invite any colleague to work with them until two years after leaving the company is a valid condition, and it contains a clear benefit for the company. Whoever enters into an agreement with them and commits to their condition must fulfill it, for Allah Almighty says: (O you who have believed, fulfill [your] contracts). The default for contracts and conditions is validity and permissibility, unless they violate the Sharia, or permit what is forbidden, or forbid what is permissible. However, if the employee leaves his work of his own accord without any influence from you, then there is no blame upon you for contracting with him afterward, because he is no longer an employee of the company.