You need to identify what process attempts to fill it up and tell the guy to stop doing it in production. The statement suggested above may not work if there are open transactions. Do "dbcc opentran('tempdb')" and act upon the SPID that is reported in the output. Do not issue KILL statement unless there is no other way to get rid of the open transaction. See if the offender can issue COMMIT so that the log can be flushed.
I agree, I was just looking to apply triage to help the dba get to the point where they could hunt down the offender. We're on the same track, and they'll want to follow your advice for the long term. I just hope that my suggestion buys them some breathing room to do that.