First, when someone first comes to you with a project, give something with your process on it in writing and your boundaries. Example (Not to meant to be comprehensive, just to give you the idea.): How CreativeCo. Works Our process involves an initial, in-person design consultation with your representative at your place of business or our offices. At that meeting, we will get clear with you on the what the final product will be, what your desired schedule is and whether we can meet that schedule. We then return to you within 48 hours a written description of the deliverables, a price and a delivery schedule. We ask that you sign this estimate and fax it or email it back to us. CreativeCo. does not work on weekends, Weekend work is not figured into the schedule for deliverables. Changes to the signed design brief must be in writing from client representative. CreativeCo. will provide a revised design brief incorporating the changes and detailing impact of the change on pricing and scheduling for client signature within 24 hours. If you do it that way, all you have to do is remind them verbally of your policy outlined to them before the project began. Much, much simpler. Failing having done that, I would simply say (and repeat as necessary), “I’m sorry that’s not possible.” Never get into explanations.