Is there any word for a person who ruins magician's trick?

A Spoil-sport is somebody who could reveal the secret to a magician's trick, as per your example, but also the type of person who kicks the ball under the bus, tells a four-year-old that Father Christmas is not real (an obvious lie) and who tips-off the teacher about the whoopee cushion beneath his seat. To accuse somebody of being a spoil-sport carries a heavy connotation that their undermining of all fun and happiness is deliberate, and that they are probably deriving some kind of depraved pleasure from the act.

Synonyms include miser, party-pooper and killjoy.

When someone reveals a magician's trick it is referred to in the community as "exposure".

Magician's logically refer to these people as exposurists.

How about a spoiler or a heckler?

Edit: There is also a Wiki article that says:

'exposure in magic refers to the practice of revealing the secrets of how magic tricks are performed'.

I am not sure how the term 'exposure' can be used as an agent noun to refer to someone who does the 'exposure'.

I like to use the words Heckler and Debunker. Also Saboteur might be the word you're looking for. I am quite fond of the word Heckler though.