When should you stop doing business with a channel partner?

It’s more than likely they’ll stop doing business with you long before you stop doing business with them. That won’t be an acrimonious act, they simply won’t be producing any sales. If they’re not making use of the resources you offer them, they’re not going to be successful and therefore are not worth your while. The more important concern is whether or not they are representing your brand honorably. If not, it doesn’t matter how much revenue they produce. Cut them loose. Your brand and your reputation are far more valuable than any particular partner.