I think merit farming should be more moderated, so it's better to report individuals who do that and let moderators check and issue a warning/punishment rather than disabling merits in a whole thread. There are many good and informational posts there so I think they shouldn't be discredited because of others.
I've seen a lot of merit farms in local boards/threads that accumulate even more merits and I haven't seen any action taken. It is probably because it would be hard to moderate every single thread and user manually so the system can't really be accurate and ideal.
How can that be enforced though, I don't know much about stuff regarding merit farms but there are a lot of MS that have threads that they open to share your posts that you think is worth the merit and it just so happens that there are some people that's active on posting on those threads so no one can really blame them if they get chosen to get those merits.