you don't even need usermanager to do this. all you need is configure transparent proxy:
[admin@MikroTik] > ip proxy set enabled=yes port=8080
[admin@MikroTik] > ip firewall nat print
Flags: X - disabled, I - invalid, D - dynamic
0 ;;; default configuration
chain=srcnat action=masquerade out-interface=ether1-gateway
1 chain=dstnat action=redirect to-ports=8080 protocol=tcp dst-port=80
and then make two access list rules (in my case they are generic, in your case, you will need a specific IP address there for the non-paying customer:
[admin@MikroTik] > ip proxy access print detail
Flags: X - disabled
0 dst-host=www.mikrotik.com action=allow hits=14
1 action=deny redirect-to="www.mikrotik.com" hits=44
The first rule allows access to your error page, the second rule blocks all access and redirects to the specified error page.