In a word: no.
Sure they could write a firmware update that blocks out the USB ports. They could write a firmware that blocks out the HDMI port, or the multi-out port for that matter, making the console for all intents and purposes, useless.
There's no point to either which would effectively cripple the hardware.
There are simpler ways of preventing an external hardware hack based upon an understanding of how a given hack works. To continue to use the hack will essentially boil down to having to use the current firmware on the PS3 and never connecting the console to PSN again, which is crippling enough itself.
Will people do this? Sure. Buy another PS3. Would it have a major effect on PS3 game sales? It all depends on quickly the issue is dealt with.
