1
How to blacklist
The crossed-circle button sits right where the heart is: on the exchanger page and in the catalog. The site asks for confirmation first. A favorite exchanger has to be removed from favorites first.
2
What happens next
The exchanger disappears from every rate table and from network hints, and its row in the catalog turns grey. Bring it back on the "Blacklist" tab of the "Favorites" page or by pressing the button again.