I am not sure that limiting how much merit you can give to a post is going to solve that abuse by itself. It might be a good idea, but it still doesn't stop someone from giving 5 merit each to 10 posts instead of 50 to 1. It has the same effect. Instead maybe limiting how much merit you can give to a certain person, or how much you can give in a single day, etc. would be more effective, though just negging people that are clearly abusing the system might already solve this before it becomes a problem.
This guy's idea might be another option for adjustment of merit system. This recommendation focuses on maximum merits each user can give to a specific user per day.
