*A situation where you might have this problem:
I have a treefile with 5000 trees in it.
I import this file into R using the package ape, so the class for this object is multiPhylo.
I enter the command "plot(trees)," like an idiot.
RStudio plots each tree, one at a time, with the text output "Hit <Return> to see next plot:"
I tried "^c" and a bunch of other things. My office mate suggested that I set my coffee cup down on the return key until it stops.
Turns out I just had to hit ESC. Duh!