It shouldn't be implemented because of what EnEsCe said, but I think if an argument occurs you should show some maturity and just walk away or just don't say anything that would start an argument in the first place.
There's a difference between a debate and an argument. Debates are civilized arguments with valid information given from each side. Most likely an argument is, or if not, will turn into an immature, spam filled, flame war.
Why would you want to completely ignore a person? The ignore feature is already there, most people just don't realize it. It's a blend of self-control, thinking logically and within reason, and not posting back at the person if you don't have a tame and civilized reply.