http://dackz.net/misc/code/serverplugin_tempent/
The plugin stops players from executing the "te" command which can spawn temporary entities on the server and crash the server. (Temporary entities are those skulls, rings, purple checkered boxes, et cetera.)
It's fairly straightforward and the relevant code is on the page. Binaries for both Windows and Linux are there as well.
I wouldn't be surprised if Valve released a fix fairly soon, as just adding the FCVAR_CHEAT flag to the command couldn't be that hard, but this should work meanwhile.