Hold Your Ground
I realize this is hard advice to take when the implied-threat of every request made to the IT department is "...or else we'll go behind your back and do it anyway..." but I suggest patience and teaching. This is where the good-will you should have been building up by explaining those "No" answers will come in quite handy. If you haven't been doing that all along, this will be a lot harder.
You need to explain the risks involved, the costs, and the total lack of flexibility "one-size-fits-all" cloud-based business apps actually represent. In simplest terms, cloud apps work great for simple businesses with a handful of people in them who don't have a lot of "business process," because in essence what business-leaders should be hearing when the words "go-to the cloud" are spoken is "change your business to fit the application." The problem with a "cloud" solution like Salesforce is that while it may fit you today, there's no guarantee the next set of changes that get shoved down customers throats will fit your model going forward. This is the inherent, fundamental weakness of buying business applications this way, and can't be easily "explained away" by a slick salesman or anxious business unit: The vendor has total-control of which "version" you're running, and is free to change any feature that they want, any time they want, in any way they want, regardless of how that affects you: Your only recourse is to cancel your account.
When I hear about business units rebelling "because IT says no!" what I really see are spoiled children, whining because they didn't get 100% their way, immediately, in every case, and now seeking a way to strike-back.
But that childish mentality will ultimately only damage the business, and leave it stuck with huge expenses owed to a variety of "cloud" service providers who have no clue about your business and may or may not be providing what they've agreed, or what is needed. And, of course, independent IT projects in each department will be far-less efficient because--hey, what happens when sales wants one solution and marketing wants another?
Answer: They both get what they want, spending twice as much money, and making it impossible to cross-train those groups. Genius!
