Isn't it odd that the gesture for "quitting" or "ending" something is using a sawing motion back and forth on our neck with our hand?
Many people intuitively understand what this means, even though we are so distant from a time when beheadings were common. Unless beheadings are common for you, in which case you probably see nothing strange about this at all.
For the rest of us, objectively looking at this, it feels a little morbid, doesn't it? However, I cannot think of a better gesture that communicates the stopping of something as well as the beheading motion.
Sure, you could make a big "X" with your arms but then you look like a confused, misplaced referee. Alternatively, you could shake your head in a disapproving "no, stop that" manner but it is arguably less direct and effective.
On the other hand, depending on your facial expression, this beheading gesture may be received as a death threat -- communicating that you do actually want to behead them. This too is unsavoury.