If this is bothering the city there's a pretty easy solution: Just make rates tiered. Charge $50 for the first mWh, $200 for the second, $500 for the third and so on so forth (just making up the tiers to demonstrate). Make it unprofitable for the miners and they'll be gone inside the week.
Not sure what the problem is here.
I don't think i missed any reply to this but to answer your question. The problem is that would be illegal for them to do that as far as I am aware. Given that you only have one electric company and zero options for power from anyone else there are laws in place for how often and how drastic the increase in power rates can be. I can't recall the exact numbers now but I want to say that they are allowed to raise rates between 3 and 4% a year, anything outside of that requires government permission. So for them to make a change like that they would have to get special permission to switch to something that much different from what they charge now.