Do you have a MikroTik as your gateway router? You can use the (free) proxy facility right in that router to do this. You will, however, need some website where you can store the message to the customer (it can be on your LAN if you have a server running 24/7).
Here is the configuration I used to present "out of service" messages to clients who had not paid their bills, and clients whose service was (mutually agreed) shut off for a season. I would just put their device names or addresses in the firewall address lists "arrears" or "standby" and all their traffic would be routed to an error screen on the server.
/ip firewall filter print
105 ;;; Process arrears...
chain=forward action=jump jump-target=blocktraffic src-address-list=arrears
106 ;;; ...or standby
chain=forward action=jump jump-target=blocktraffic src-address-list=standby
136 ;;; Allow any DNS, NTP
chain=blocktraffic action=return protocol=udp dst-port=53,123
137 ;;; ...or (proxy redirected) HTTP
chain=blocktraffic action=return protocol=tcp dst-port=80,443
138 ;;; Don't log on EVERY packet
chain=blocktraffic action=reject reject-with=icmp-network-unreachable src-address-list=throttle-notice
139 ;;; Log arrears...
chain=blocktraffic action=log src-address-list=arrears log-prefix="ARREARS"
140 ;;; ...or standby
chain=blocktraffic action=log src-address-list=standby log-prefix="STANDBY"
141 ;;; No more logs about this user for 10 min
chain=blocktraffic action=add-src-to-address-list address-list=throttle-notice address-list-timeout=10m
142 ;;; Block all other subscriber traffic
chain=blocktraffic action=reject reject-with=icmp-network-unreachable
/ip firewall nat print
11 ;;; Arrears notice
chain=dstnat action=redirect to-ports=8081 protocol=tcp src-address-list=arrears dst-port=80,443
12 ;;; Standby notice
chain=dstnat action=redirect to-ports=8082 protocol=tcp src-address-list=standby dst-port=80,443
/ip proxy access export
add dst-address=999.999.999.999 dst-host=somewhere.somewhere.xyz
add action=deny comment="\"Pay bill\" notice" dst-port=80,443 local-port=8081 \
add action=deny comment="Seasonal standby notice" dst-port=80,443 local-port=8082 \
add action=deny comment="Secure the proxy"
/ip proxy print