The answer to that is you apeal the ban or report your house mate's main for the trolling/harassment before the perma ban, which as a GM you should be for as that promotes a good playing enviroment.
In general I think IP banning isn't practical.
I was told for some Perfect World game that my IP had been used for some kind of bad behaviour, so I couldn't use some of their web services. I had only just started that internet service recently. Some kind of PC ID in combination with other factors would be best.