As some of you may have seen an announcement on it, I am joining the team as an Enforcement GM. I figure this is a pretty good topic to start with to weigh in my two cents.
Here is my take on the matter. Any skills/spells/items that purposely prevent someone from fighting the MVP is griefing. Trying to excuse it by saying you are doing it for a competitive advantage or to help your party is just trying to avoid the point. It is still griefing. This question in particular says a lot; "Is it allowed to SW an MVP so an enemy champion cant asura, or pneuma for snipers?" The answer should be obvious to those who understand the concept of griefing. For those who it is not so obvious to, the answer is no. Paraphrasing the rules here "griefing MvPs will not be tolerated", is the grounds for this answer.
Also to note, you guys should consider it friendly competition, not a battle against enemies. That is what WoE is for. Calling a player an enemy and going out of your way to make their life harder is considered both griefing and harassment. This is a surefire way to get yourself jailed or banned. Now if you happen to SW (Safety Wall) one of your party members to protect them, and it's close enough to the MVP to stop damage to him as well, that can be seen as a mistake. Doing it just to stop someone else from killing it will never be acceptable.
If anyone has further questions regarding what is and is not considered griefing, I would say to first think about what it would feel like for someone to do it to you. Then if you are still confused, ask about it. I am sure people can give you some good answers, and I will do my best to weigh in should there be confusion on the matter.
Thanks and best regards,