Is it legal to bait or troll a sting operation?

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?

If it was illegal to chat up kids then Michael Jackson would be alive and well and living in prison.

I have a feeling the cop would realize this guy was stringing her along. Undercover cops are usually smarter than that. Furthermore, let’s say he was “harassing” the undercover cop. I doubt she would blow her cover just because the guy was harassing her. If she wants to keep busting Johns, she needs to keep it low key. She’d probably just tell the guy to get lost, keep walking away. If he follows for a bit, reiterate that he should get lost, keep walking. Perhaps threaten to call the cops if he keeps stalking her (that way she’s still got her cover.)

Yes, it is illegal. It’s called “obstructing a police officer”.