Restrict use of static / manual DNS to connect internet?

is it possible to restrict the clients from using custom DNS to pass through the router's specified DNS ?
i have installed pi-hole on a linux server to block some queries, but it is useless if the clients is using custom DNS

my router's ip is and my linux server is

Catch all outgoing traffic on port 53 UDP and TCP in your FW and redirect it to your pihole.