Let's say a government tried to enact a law to prevent 51% attacks, how do you regulate it?

"You may go no higher then 50% of the Global hash rate"....

In today's centralized hashing environment it could create "hash wars" to try to eliminate competition...

Todays hash rate % example:

ghash.io = 26%
Discus Fish = 17%
BTC Guild = 11%

BTC Guild and Discus Fish decide they no longer wish to compete against ghash.io, both simultaneously shut down for lets say 10 blocks, which would probably take less then an hour with a 28% global hash rate drop. Then they fire back up.

ghash.io clearly has more then 51% of the hash rate for that hour, what then?