3rd side to the coin (Score 1)

As long as you didn't send the resignation on the tip of a flaming arrow stuck in the back of your former boss... no you did nothing wrong.

However, the attitude that people have zero value to the organization as soon as they give notice is a bit short-sighted. Too many times have I seen heavy-handed tactics work to cause more damage then they prevent. I have never worked in an IT dept that wasn't understaffed and that means that every one is _the_ expert at something. & when was the last time documentation was up to date? You want to get as much of that knowledge transplanted to the home team as possible. By treating soon-to-be-former-employees like criminals you've poisoned the relationship making it nearly impossible to get anything useful out of them.

Yes, things have changed and you must protect your systems and data. Explain to them you're going to have to restrict their accounts bla-dee-blah-blah. Keep it low key & offer an easy carrot or two like references or some such. But keep them talking on topic & to the right people. It's not all hugs & kisses but enuf knowledge usually gets over so that the switch in responsibilities is smooth(er).

It's usually easier to deal with one person leaving than 3 or 4 trying to figure it all out... all over again.

Now excuse me I have an arrow I need to deliver