Wildcard support to block all of domain.com and *.domain.com is an as of yet to be implemented feature request. Be sure to vote for it in the feature request section: Wildcard support for domain black/white listing
Right now all you can do is customize your installation manually to block all of a domain, check out this post:
https://github.com/pi-hole/pi-hole/issues/672#issuecomment-247863064
(also be sure to service dnsmasq restart / systemctl restart dnsmasq afterward your manual config modifications)