convince the big miners? you could just lock their accounts.
also you should make 2 pools for LTC and spread them.
locking any miner account won't help, even based on the given account total hasrate : miners could just split their hashrate over multiple accounts, recreate them when they wish to.
IP-based bans are useless too, anyone can get dozens of new ip addresses at will nowadays.
The whole 50+ % thing isn't coinotron's fault at all, but THE MINER'S !
miners should be aware of what they're doing, regarding the mined coin...
miners could for example :
* setup their miner to switch to other pools at regular intervals
* setup a damn p2pool node locally and start mining there, either full time or using your miner app capability to cycle between pools automatically