Please add a ‘Smack’ option in multiplayer so you can smack down teammates who betray you. This would stop players betraying each other just to get power weapons, leaving only the chronic grievers to deal with.
You could get the MM voice guy to shout out something fun like “SmACKdown!”. Asking for “You bin SLAPPED, fool”, something Charlie Murphy-related (“FIVE fingers!”), or something Barney Stinson-related (“Slapsgiviiiiing!”) is probably too much to ask… 
The won’t be stopped. Just like little kids won’t be stopped from playing games they shouldn’t. It will happen. Betrayers will betray and haters will hate. All you can do, is report it when it gets to be too much and move on.
There must be a better way to detect betrayals and even griefers. For example there could be a behavioral script that monitors a players actions; if he/she keeps beating/shooting a teammate just when their shields are recharging or just finished recharging then it would attribute that to griefing and place a red flag on that player. You know? These types of conditional monitors aren’t that hard to work out.
The smack option is simpler since the victim gets the option to smack or not. If it’s an accidental betrayal, I won’t boot/smack someone given the option unless it was REALLY dumb on their part.
Betrayers will betray but smacking them down would make me feel better. The reporting system is completely useless and ineffective and the Xbox Live avoidance system is worse.
They are not getting rid of them, but they should be toned down, ALOT. Especially grenades.