http://linux-audit.com/how-to-clear-the-arp-cache-on-linux/

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root@ubuntu:~# arp -d 192.168.1.1

After deleting, have a look with the arp utility again to see the new list:

root@ubuntu:~# arp -n
Address                  HWtype  HWaddress           Flags Mask            Iface
192.168.1.1                      (incomplete)                              eth0
192.168.1.2              ether   00:02:9b:a2:d3:f3   C                     eth0
192.168.1.3              ether   00:02:9b:d9:d1:a2   C                     eth0

Clearing cache with ip

Newer Linux distributions have the ip utility, which has a more advanced way to clear out the full ARP cache

root@ubuntu:~# ip -s -s neigh flush all
192.168.1.1 dev eth0 lladdr 00:a1:04:c6:10:14 used 757/757/28 probes 6 STALE
192.168.1.2 dev eth0 lladdr 00:02:9b:a2:d3:f3 used 2555/719/659 probes 6 STALE
192.168.1.3 dev eth0 lladdr 00:02:9b:d9:d1:a2 ref 1 used 0/0/0 probes 6 DELAY

*** Round 1, deleting 3 entries ***
*** Flush is complete after 1 round ***

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