Unlikely to be able to induce enough current in a fan induction motor with a vacuum cleaner to do any damage with the computer switched off, but blocking the rotor by folk not knowing all the possible consequences is risky, depending on what is used to do the blocking, bearings can be damaged, blades and motor shafts broken or bent, causing rotor imbalances, the fan is usually quite a delicate mechanism, it isn't designed to withstand being physically touched by objects any more solid than air.