[jmull]

If you reduce it by 50% you save 50% waste. If you reduce it by 75% you save 75% waste. You dont' have to entirely wipe it out to save a lot of waste, just make it harder and/or more costly.

Attack the supply side, attack the demand side: tax it, regulate it, cap it, replace it. All of these can reduce the mining so they are all useful.

edit: to answer your idea that they will just move...

If these other places were as good or better for mining than where they are now, they'd already have moved. If you make them move to a worse mining situation, that's just proof that what you're doing is working.