What do you mean, "at the expense"? They went off on a tangent, they were not mocking the prompt, or the prompter. This is not a difficult concept to grasp.

Personally I like it when a conversation (or even, gasp, jokes!) start up on one of my prompts, because it keeps it visible in flat view, and makes you more likely to get a fill, and I doubt I'm the only one.

If the OP doesn't like people interacting in their prompt thread, they can always take it to the mod. Or I guess this time you can do it on their behalf if you feel so strongly that no one should have fun here ever.