They should just up the team damage to 100%...and make team kills an automatic 2-3 points of the 5/5 report. Putting the damage to 100% would really deter people from kill-whoring...because 5v1 will definitely do more damage to your own team than the damage you do to the enemy resulting in loss of the round and any multi.
You really believe the mass of people would understand it that way?
If I see a 4v1 I already stay away and look for something else... and still I see people going that way to make it even 8v1 and then they're wondering that they're getting teamhit. People are stupid. Unfortunately they even seem to refuse learning out of experience.