51% attack + ignore blocks not mined by yourself 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)
Yes. The owner of 51% can get 100%. This is attack-51 itself.


  Is there some measure that would stop this?


no