I don't see it expressly stated in the book, but my interpretation is yes. The String is spent. The text says "You can spend a String to..." You don't get the String back on any of the other things you can spend them on, so I don't think this option is any different.
Don't think of it as spending a String to force a PC to do what you want. Instead think of it as spending a String to get the chance to offer them XP to do what you want. Something you normally can't do.