Could be virus or a hacker. There is a worm on the net going around in regards to SQL. I suggest you get in touch with the Admin of the other network and have them drop that user until the problem is corrected. Explain that you must consider it an attack on your network and that it must be rectified. Or if you have a firewall between you and the remote network block the port for SQL unless you must have that open, thenn filter the remote network out. Lastly if they are unwilling to cooperate and they are in the US check with your comapny attourney what recourse you may have if it becomes a major concern.
"Don't roll your eyes at me. I will tape them in place." (Teacher on Boston Public)