How to keep plans for a job change quiet?

I am most likely leaving my permanent position at my current university for a permanent position at a new university. I have been given an unofficial offer and we have agreed to terms. I have been told it could take over a month to generate an official offer and contract and that I should not give notice until then. I would prefer not to broadcast the news widely since nothing is set in stone until I am given the contact. Obviously I do not care that much since I am asking here.

In the few days I have known, I have told:

• My current department chair since he wrote a reference and my other references


• A couple of close colleagues since I valued their opinions


• A collaborator in my department since we were beginning to to write a grant together since the move, it is international, would cause major problems


• I am now faced with having to turn down a prospective graduate student which would require me to tell at least the head of our graduate admissions committee and maybe the whole committee

I am clearly failing at keeping it quiet. How do you not screw over your colleagues, but still keep the job change quiet until it is official?