That would work OK for one employee, but it sounds like a lot of people are doing it.
Let's see - start with changing the extension he's using. Set up an IVR on the old extension. All forward go to IVR - direct calls result in suspension/firing.
From there, you're choices are pretty much endless. How you proceed is actually going to depend on how your CEO is supported. If he answers his own phone, then you want some kind of automated assistant in front of him. If he has a assistant, you can set up the phone (if you can use PJ-SIP) to be a "second line" on the extension number and simply set his to not ring. There really are lots of ways to handle it.
When you set up his new phone, be sure to set if internal caller ID to the old number. That way people will still get confirmation that his "number" is the same as it used to be.
Another idea would be to set up a series of steps that starts with another process ("Announcements" for example) and goes through the steps you want to make happen.
Unfortunately, there's no way to really give you a lot of support, since you certainly want something to happen, but you haven't told us what. For example, with your description, the right answer could be "take away his phone."
Let us know what you want to do and how you want to do it. We can probably help.