I think Fidelia has characterized the dynamic pretty well here. All I would add to it is that an ENFP will hear the INFJ's "back off" message, but will decide to ignore it because:
a) the reasoning of the INFJ is flawed (which is what Fidelia said),
b) that to submit to the INFJ's request would somehow violate an Fi value dearly held by the ENFP, or
c) the INFJ tries to communicate "back off" by avoiding the other person in some way... maybe not a complete door slam, but more of a withdrawal. The ENFP recognizes the retreat but doesn't understand what the INFJ is trying to communicate. In fact, withdrawal might even incent the ENFP to approach the INFJ. In other words, not only does the INFJ not effectively communicate their needs to the ENFP, but also they incite from the ENFP the very behavior they are hoping to avoid. <-- I'm going to bet this third scenario is pretty darn common.
So what's the answer here? How can ENFPs and INFJs (who are by all accounts friendly and kind people) avoid getting in such an unhealthy dynamic with one another? If we could figure this out maybe there'd be less doorslams going on out there in the real world and I think that'd make both ENFPs and INFJs happy.