They might feel extremely uncomfortable as most posters had already mentioned under this thread--it can stress them out since they probably don't know how to put it into proper closure without hurting the confessor's feelings.
If they still feel the emotions remain strong toward them, then they'll probably resort in emotional-distancing. When everything fails, they might resort in doorslamming--not to hurt you, but to help you to get over it.
In my case, whenever I've received signals that someone like me more than as a friend when I have no mutual feelings back in return, I often do try my best to offer obvious hint to them that I have a crush on someone else but them (even when I have none). If that didn't work out, I might apply the above two mentioned steps.
Most people didn't realize that having an emotional burden can be extremely daunting for INFJs. First of all, I think most INFJs don't fall in love easily and once they do, they tend to fell hard. What is more, it can be quite challenging for INFJ to accept the change in human interaction, especially when it comes to one-sided romance. I've had few similar experiences among my coworkers and I just had to change my pattern to either leave work early and avoid hanging out with them. Usually they can pick up the signals really fast and when I felt they didn't have any romantic attachment toward me, I can easily let them back in my social life.
However, I often felt that when INFJs like someone, they can be a little clingy. Or creating more opportunities by organizing social activities just to get closer to their romantic interests.
Well, at least that's how I've been.