I don't know how it works but I think there is a bot on Steemit "@cheetah" that detect plagiarism, thus developing a similar bot wont be a problem (there are many senior developers in this forum).

It will be great if you succeed to write a script that detects members sendig Merits to each others.

I don't think it is going to be hard to code such script but you will need an access to the Merit database.