Both of the 5th dans in my dojo, when I've asked them how to handle a technique when uke is not doing what he should (eg, not grabbing the shoulder in kata tori, doing more of a yokomen than a shomen, etc) tell me that "uke is being stupid" and to do another technique. When they teach and their uke is not attacking with the right energy or direction to do the technique they are trying to demo, they often will just do something else. So I know why so many are talking about free-style, but I don't like it...I would rather do the technique that we were just shown.
Depending on the ukemi level of my obnoxious partner, I will a)ignore it, tomorrow is another day.
b) tell them I would get more out of it if they would XYZ rather than ABC.
c)give them 12 chances to behave, and then shut them down in return (usually gets their attention as I'm half their size, and their ensuing struggles often attract the instructor's attention)---make sure your ukemi is up to this before you try it.
d)if they are still obnoxious, and I know their ukemi can take it (around here usually means shodan or higher) I do an alternate technique based on what they are forcing
One other thing, sometimes resistant ukes are frightened ukes, so I will ask that if a) I think that is really the reason or b) I know it is not but they are too obnoxious for words.