jcwhitney might not be the seller's I.D.
I have tried blocking them twice now using that I.D. and my searches still come up with pages full of their product catalogue
Did you save your new searches? They have been gone from my searches since excluding them. What I did was this;
opened a search
scrolled down to more filters, which opened a window
selected seller from the list
checked the box "only show items"
checked specific sellers
typed in JCWhitney in the box next to Exclude
hit Apply
saved the new search
JCWhitney is gone from search
Is this what you did?