If you own 51% of the mining power and decide to ignore all blocks not mined by yourself.
You would eventually end up having the longest chain, and would eat up the 49% of block-reward you werent getting prior. And now control 100% of the blocks, and could censor transactions etc, since any blocks not mined by you would eventually be a stale fork.
Is there some measure that would stop this?
It seems if you control 51%, it would be in your economical interest to ignore blocks not mined by yourself. (other than it being bad publicity for btc, and maybe crash the price)