Figure 1
This tutorial will guide you to block udp ports 1024-65534 in a cisco 2811 router.
As in figure 1, add an ACL rule in "Firewall and ACL", and set the destination port to range "1024-65534".
If a packet does not match any ACL rule, the default action is "deny". So you also need to add "allow" rules for all other packets.
