I want to know if a merit given to a post can be reverse or revoke. May be if somebody give me a merit can the person take the merit back if he thinks the post don't deserve it.
Merit cannot be taken back. This also keeps in check any possible merit farming rings that may start. But on a second thought this might be a good thing to implement in future because there can be accidental merit award happening by people who are in a rush or simply by accident. Then again the cons are outweighing the pros.
They are all right, you can't reclaim a merit/s once it has been given to someone. Look at one of sir Vod's post as an example: https://bitcointalk.org/index.php?topic=4522471.msg40823682#msg40823682 . So if you are the receiver of the merit not intended for you then cheers, you're a lucky one 
.
Thats exactly what some shitposters are having wet dreams about. .