As for the sentence, 'there's no such thing as ghosts.' Is it grammatically correct to use the plural form of ghost? From what I have learned, the object 'thing' is in singular form, so ghost should be singular too. But I see a lot of native speakers use this sentence.
There are no such things as ghosts.
There is no such thing as a ghost.
are both grammatically correct.
There is no such thing as ghost.
is completely incorrect but a common mistake among Chinese English-speakers (i.e., Chinglish) because Chinese does not usually distinguish between countable and uncountable nouns the way English does. This sentence would mean "there's no such thing as the substance (物质) known as 'ghost'". Since that doesn't exist, the sentence has no meaning.
There is no such thing as ghosts.
on the other hand is perfectly fine as informal (不正式) English because—even though its grammar doesn't agree in the logical way prescribed by English grammarians—it's actually clearer phrasing. The phrasing of There are no such things as ghosts and its singular counterpart focuses the listeners' attention on the number in a strong way. The speaker, on the other hand, isn't concerned with the number of things that don't exist but with expressing the complete nonexistence of an entire category of things.