I'd be asking them for a break down for the past few months usage, and what the actual issue is. Seems ridiculous to boot someone off for one offense. The "three strikes" would make more sense. Also get it in writing that they've booted you out and the reasoning just in case their account system is stupid and keeps trying to bill you.
Someone mentioned Voyager, I've been with them for a while, and have had no issues or warnings about usage etc - doing approx 1TB a month.