In Iptables, Accept incoming UDP traffic to port 53 & reject everything in the port range for static ports.
The highest limit (here 32767) should not be too high otherwise, your server will be unable to resolve external domains (for instance when you do a "ping google.com") from inside your server. On a linux OS, 32768 is the first ephemeral port for dynamic sockets up to 61000. Thus, 32767 is the highest port for static allocated ports.
-A INPUT -p udp -m udp --dport 53 -j ACCEPT
-A INPUT -p udp -m udp --dport 1:32767 -j DROP
To get your local dynamic (also called ephemeral or private) port ranges for UDP & TCP. FYI, Linux OS aren't compliant with IANA. :
Then, if you want to open a port for UDP like a VPN port, just add an ACCEPT like before the DROP line.