This is why every competent HR department sets up a strict semi-automated termination procedure in coordination with the IT department that locks the accounts of anyone receiving notice of termination regardless of cause before they are notified.
This includes locking out their phone, email, remote access, and even removing any work laptops or pagers. An expensive lesson in how not to fire someone.
On occasion, I waive the two-week notice immediately, such as if I suspect the employee is moving on to a competitor or may be disruptive to the other employees. Employee still gets the two weeks of pay but he/she is out the door in minutes.