/support for instituting a level cap for kill credit.
/no support for any kind of penalty, shield, etc.
People should be able to kill players 10+ levels below then, they just should not get rewarded for doing so. My reason for this is the same as the reasoning for a level 70 not gaining exp from killing level 60- mobs: rewards should come from challenges. You kill a 70+ mob with a level 70 toon, you gain exp because there is some challenge to it. You kill a level 80 player with a level 80 toon, you're on somewhat equal footing, so there is some challenge to it.
As for low-level grinder killing being the best or only way to rank, what then does your rank prove? Not that you are a good pvper with a good build and set. And besides, the time of day I usually play is generally really slow pvp and grinder-wise. The only way for me to rank.up fast would be dual-box stat padding. Should that, then, be acceptable?
Fight people your own level if you want to rank. Trying to justify ganking lowbies as pvp just makes you look sad and pathetic.