Are you the admin on the box, I mean, do you have FULL root access to the server? If so, a program I run on most of my external facing servers is DenyHosts. Its really cool for a couple of reasons.
First, it monitors all incoming connections, if an IP address exceeds a specific threshold, say 100 connections in a minute, it places the IP on temporary ban list. If the IP continues to try and connect after being temp banned, it gets moved to a permanent ban list. The difference being that the temp ban list gets purged on a set time-table.
Second, all bans happen on the hosts.deny level, so the service the IP is requesting isn't even bothered.
Third, and probably greatest of all, is that the service communicates with a central server where others running HostsDeny share offending IP addresses. You can optionally download and also ban offending IP's others have permanently banned also.
After this service runs for about a week, you will find there are VERY few unwanted connections coming through. I went from around 10,000 various malicious connection attempts a day to less than 5 per week.