A320baby - I would beg to disagree. In the same way that we are given dire warnings here on PPRuNe that we are not contributing in a vacuum, no employer can be immune from facing the consequences of sending threatening or abusive e-mails to its employees. It is a key protection of employees to post unsavoury communications from their company should the need arise. This is not commercial-in-confidence stuff - it is a straightforward and direct threat from their management.
Now regarding whether it is reasonable or not, that is very hard to say. I have no idea what company is involved here, but in principle it would seem reasonable for a company to expect someone to serve out their notice without either leaving early or becoming so belligerent as to make themselves unemployable. Equally, some sort of slave labour is unacceptable too. The key thing here is to get to the bottom of what is going on in terms of employee or employer activity that has lead to this impasse. I would also add that another factor here is that my understanding is that it is not legal to retain employment records, security details etc from past employes with a view to preventing them work in the future. Therefore, I think there are legal considerations here. Whatever the truth of the situation, I would certainly not want to work for employers like this.