Here's another potential user/manipulator situation--certainly not to the degree that SS Ellen was a manipulator, but enough that I'm on my guard with her. This is a co-worker of mine who has a history of abusing/disrespecting my time. She started out with having me type up a few things for her and worked up to pawning off a major project on me that she didn't tell me would be as time-consuming as it turned out to be. I did it for her for one school year and then said "enough." Well, here's what I actually said, although same idea
"When you asked me to do Project X for you, I didn't realize it would be an ongoing thing, or that it would be so time-consuming. I was happy to help you with it last year, since we had BigEvent coming up, but I won't be able to do it on a regular basis. I have projects
I need to work on for MyDepartment that are higher-priority. I'm sorry, but you'll need to make other arrangements for Project X this year."
This is J., the supervisor of K., the co-worker who asked me to give an oral presentation for her clients--the same one who tried to guilt-trip me into doing the presentation because it was good for teeeeeeeeeeeeamwork.

I suspect she told K. to ask me to do it "because Waterlight never says no." I'm all for teamwork--but not at the expense of higher-priority projects in my own department, or of my leisure time.
Last fall, J. asked me to substitute-teach a class since one of the regular teachers was absent. Now, if I'd been asked to interpret Lavenderese, several days to a week in advance, I might have agreed to do that--but she asked me to substitute-teach a group of toddlers, and with only 20 minutes' advance notice! The short notice alone would make me reluctant to do anything of the kind. But this is not something I'm even qualified to do; I've had no coursework in early childhood education and minimal experience with children in this age group. We require even our subs to have had at least one year of work experience with young children. So, my response in this instance was the E-Hell-approved "I'm afraid that won't be possible."
She did some PA sighing and said "Well, I guess this means I'll have to teach the class myself and hire an interpreter."
I said, "I guess so, since I have a prior commitment I can't break."
I question, though, how much of this is deliberate manipulation and how much of it is simply failure to plan for unexpected contingencies. That was not the first short-notice request I'd gotten from J.'s department.