multi-home High Sierra and block one address?
The easiest way to achieve a guaranteed reject with no timeout is to use a broadcast address 255.255.255.255. Edit your /etc/hosts file and for hosts that you want request rejected put an entry such as:

255.255.255.255  rejectedhost.com
255.255.255.255  someotherhost.net


This free and simple technique can also be used to block out advertisement hosts on ad-ridden or popup-ridden web pages.