No I don't believe it is a full ban. Whatever it is, it is very strange. For instance, my FLGS, a huuuuge place, has stacks of Reaper Kolinsky Sable but I ask them to order some W&N and they responded that their distributor was out of stock. So I search online for USA stores that carry W£N and they all have extremely limited stocks, as if they have not re-ordered in some time. But I could find surpluses of some other brands.
So something is going on, but there is not a ban. I ordered my W&N from PK-Pro, by the way. There was a custom form attached when it arrived, so it was looked at and not confiscated.
The he weasel that KS is made from is from Siberia, if I recall correctly. Though I have heard that this weasel is actually not even used in modern day KS brushes. You can order just the hairs alone from China. I'm curious though and would like to know the intricacies of this situation.