Even given enormous flexibility, I still have problems with work. In college, I had a terrible experience with a stalker on the campus safety force. I nearly had a nervous breakdown, and I needed a couple of days to recoup. My employer reluctantly gave me two days off, but then decided that those two days meant I wouldn't receive my stipend (less than minimum wage, but I was receiving college credit) for the entire week. I don't think she meant to be difficult, but it was no way to respond to an employee who was having a very difficult time. I didn't know what to do about it. However, I began to be less flexible, and reminded her that in reality I was only supposed to be there for 10 hours a week, not the 30+ I had been doing.
I learned the hard way, through an awful summer intern position, that I needed to let people know about my religious convictions up front. Mostly because I was treated badly because I had to leave a firm event at sunset Friday. And also because people seemed to relish in giving me projects that I could "just put a Saturday in" and be done with.
I don't think that every instance was malicious. But I do think that for some reason, being young and unattached seems to signal that you are constantly available, and that your time is completely open to your employers discretion.
As a solution to this problem, I have learned the importance of drawing lines, and sticking to them. That is the only way to avoid working 24/7 at someone else's discretion. And, as both a work principle and a life principle, I have learned that you must teach people how to treat you. That doesn't mean that you will greet the individual who attempts to upset your planned vacation with curses. But it does mean that you will press the person for other solutions to their needs. Technology will allow you to work in many places, and a discussion about the real time needs of the project may help the person get a better idea of how their Saturday night demand is arbitrary.
On the off chance (ha!) that you meet someone who really is purposefully trying to upset your life, you must also teach them how to treat you. It is highly unlikely that you are the only person at your job who knows how to do what you do. Provide them with their options. And, it is equally as unlikely that the document that you are working on must be finished on a Sunday (most clients aren't in on Sunday, and the post office/ups/fedex doesn't even deliver or pick up). Forcing the person you are working with, politely, to acknowledge the fact that weekend work is really for Monday, and that you can finish the assignment just as easy on Monday morning will work in your favor. You can gently guide that plan buster to the realization that your time is more valuable, and that you are productive enough to have a life.
Releasing your life to someone else's whims will ruin it. Maybe this is what happened to the individual who is trying to upset your life. Maybe they don't want to go home. Maybe they don't have anyone who they want to spend time with. That doesn't have to be you, and you need to show the people in your life they matter, and the purposeful "life busting" fellow employee that you will not let this happen to you. But do it in a nice way, so that the person won't know what hit them! Remember, the ultimate goal is always to teach the person, politely, how you want to be treated.