I find this question interesting, but since the argument over whether this-or-that online chat would even be legal in the first place is confusing, I propose a slight change in circumstances:
Let us presume that there is some situation where an undercover officer has disguised herself (or himself!) as a prostitute, and a potential john has realized something is up and decides to just string her along, never quite stepping over the line of negotiating sex but playing the part well enough to still be thought a mark.
I suppose at the very least, the officer could decide he’s not worth the time and tell him to get lost, at which point if he continues talking to her he could be arrested for harassment?