What is a person who fakes responses on a survey?

This procedure excluded 24 junk responses from further analysis, since they did not give honest answers.

junk
2. anything that is regarded as worthless, meaningless, or contemptible; trash.
— dictionary.com

Note: I think the emphasises that it is the responses not the answerers that are discarded.

"Outliers" might work well as a neutral term in your example sentence:

This procedure excluded 24 outliers from further analysis, since they did not give honest answers.

An outlier is "a person, thing, or fact that is very different from other people, things, or facts, so that it cannot be used to draw general conclusions." (Cambridge Dictionary)

An outlier is also "a statistical observation that is markedly different in value from the others of the sample." (Merriam-Webtser Dictionary)

"Outlier" does not suggest fakery, of course, but the context is supplied by the rest of your sentence.

It depends rather on the motives of the person (and one’s attitude towards them).

“Spoiler” might work if the intention was to invalidate the survey.

“Privacy advocate” might be appropriate if the survey purported to be anonymous and the person suspected that it was not.

“Feedbackphobe (or, US, feedbackfobe?)” might be a candidate neologism if you regarded it as a psychological condition*.

And if you knew nothing about the motive, you could always try “liar”.

*FOOTNOTE

I now find that “Feedback Phobia” (British and US spelling) does exist (e.g.in the book, Management Intelligence) but seems to be used to describe someone who fears receiving — rather than giving — feedback. Clearly more thought needs to be given to inventing an unambiguous term — something like “Questionnayer” perhaps.

This procedure excluded 24 "fraudulent responders" from further analysis, since they did not give honest answers.