show page to disabled clients

Staff, how are you?
My name is Yair Villar and this is my first Topic, hope i can get an answer or idea to my fix my problem.

well i have an ISP and i want to:

if client is disabled, redirect all trafic so that the user see my page instead of the internet page,
if client is enabled, then show normal traffic.

i create a lease for each client, then that creates a queue for each lease.

if more info is needed, let me know.

(i was thinking that the solution will be to use a proxy, am I in the right track??)