I've always been told to use a toothpick to hold the fans still when cleaning an electronic device with suctions (or the reverse) to ensure that fans don't spin past their intended RPM. Of course, this begs the question, where are the fans on the PS3? Which ones can be reached and secured (w/o opening it)? The most obvious one I can see is on the back of the unit. I think one could get a toothpick, or something smaller yet strong enough, to secure the fan.
There's only one fan in the PS3, and you certainly can't reach it without opening up the unit; it's not on the rear of the unit, it's in the center of the unit.