access-list 101 permit udp any any eq <port>
access-list 101 deny ip any any
Then apply it to the interface(s):
int <if_name>
access-group 101 in
"access-group 101 out" might work better. You'll need to consider your requirements.
The "deny ip any any" isn't really needed, but I like to add it as a reminder. As soon as you create an ACL there's an implicit "deny" at the end.
Note that this will whack ALL IP traffic inbound (or outbound) for the interfaces on which it's applied. Be very careful. For instance, if you apply this to a remote router's serial i/f, you'll lose your telnet connection.