first, find out what ip address (the website that you want to blok), using "TORCH", and put ip on firewall filter, drop forward action for this ip address.
Use the hotspot with webproxy, first enable the proxy and after in the NAT section put the next rule to redirect all TCP Traffic from port 80 to 8080 "webproxy"
/ ip firewall nat
add chain=dstnat protocol=tcp dst-port=80 action=redirect to-port=8080
Something very import is select the option TRANSPARENT PROXY into the user profile hotspot. Finally add the rule to block the website into the webproxy section.