Suppress specific SQL Server log warnings and messages
Overall, suppression or inclusion of logging is generally accomplished via trace flags - you are correct. 

This, however, does not mean that:


There is a trace flag for each log item
The trace flags are publically documented
The trace flags don't differ through versions



  such as when a spid is killed


There isn't a trace flag to turn this off, and I highly doubt that if there were one it would ever be documented or given out.

If the killed session messages annoy you, filter them out as Aaron has stated. You can create your own noise words list or known message list that you don't care about specifically for your dev environments.

Additionally, I'd look into giving them some SQL Azure databases to let them do development so that I didn't have to worry about this kind of stuff and could focus my time on actual production issues or new projects.

Disclaimer: I work for Microsoft.