I've just watched an excellent video presentation by Des Traynor (Co-founder and Chief Strategy Officer at Intercom) on product strategy and about saying "no". The video is both entertaining and informative. I understand that this presentation was part of a competition in MIT in 2013, from which Traynor was disqualified for using too many slides. Apart from that, there are lessons for all Project Managers from Traynor's thoughtful musings.
In my Project Management classes, one of my favourite statements that I make is in relation to scope creep: "If it is not on the Scope Statement, don't do it!". Change has to be controlled by developing processes to deal with change requests and managing its effect on the project - but sometimes the best way to control scope creep is by just saying "no".
"No" can be a wise word!