Two suggestions :-
1) arpwatch
Suggest at least temporarily stop logging arpwatch messages by adding a file to /etc/rsyslog.d (see below ). This will stop the clutter in /var/log/messages and and prevent the excessive disk activity as well as saving disk space in /var/log directory (Nowhere near full I hope)...
2) rsyslog
As I understand it the "actions" are numbered sequentially from the start of the "#### RULES ####" section in /etc/syslog.conf (if this is incorrect please indicate how they are numbered). Can you please show us your /etc/rsyslog.conf from "#### RULES ####" to the end (in code tags please).

Create a file named, for example, /etc/rsyslog.d/arpwatch.conf with the following contents similar to the following

See previous discussion in these forums regarding these files...