Well it is a built in Quake 3 command.
QUOTE("http://www.planetquake3.net/tweak/glossary_commands.html")
"r_subDivisions" - This is a texture setting which when set low, produces a better image with more curves and less graphical inacuracies. The maximum setting I would recommend is around 100 depending on the map. I like this set at 4 but feel free to experiment for the best comprimise between image quality and framerate.
Looks like the player is trying to use a game exploit as a wallhack. This isn't a cheat, its just a game exploit. If you want to prevent people from changing this value, just use a PB CVAR to limit the values players can use. Its really not that big of a deal, the effects of this command are minimal anyway. Game exploits aren't cheats and shouldn't be banned for since they are built in Quake commands and should be available for tweaking. If you have a problem with it, run a PB CVARs. This just looks like an idiot trying to squeeze every little bit out of his monitor....who just doesn't know how to tweak lol.