Certainly, for the case of canola in western Canada, it would be exceedingly difficult to guarantee GM-free canola grown in that region. I would almost say that at a commercial scale it might be impossible to do that, because there's so much of it throughout the system.
For other situations, other crops, where that's not the case yet, that can perhaps still be a possibility. But within a given region, as you increase the scale of a certain GM crop being grown, it becomes more and more difficult to maintain segregation and to meet really low thresholds, especially if it's an out-crossing species where pollen movement can be relatively long distance. Maintaining an absolute zero becomes increasingly difficult. For canola in western Canada currently, I wouldn't attempt it. I think it would be very hard to do.