Depending on how nice you want to be to the client, a possible solution can be:
iptables -I INPUT -p udp --dport 111 -j DROP
or
iptables -I INPUT -p udp --dport 111 -j REJECT
Depending on how nice you want to be to the client, a possible solution can be:
iptables -I INPUT -p udp --dport 111 -j DROP
or
iptables -I INPUT -p udp --dport 111 -j REJECT