Well, you can do all sorts of stuff if you have >50% of network CPU power.
This wouldn’t even require 50% of the network CPU power. 20% would be enough to “slow the network down” with a significant degrading of service and ramp up the difficulty for the rest of the network considerably. Then again, you would be a fool to pass up on potential transaction fees simply by only accepting transactions that would permit a high fee or an impossible fee.
My understanding of this is really muddled and I’m not not really sure how the fees are decided or what happens when you type in an address, an amount, and try send some bitcoins to somebody.