After Enslin receives the ominous "Don't enter 1408" postcard, it's never brought up again, not even with Mr. Olin. What would he have had to say if Enslin had shown it to him?
Probably something along the lines of "Good advice. You should listen to it." Assuming the room itself somehow arranged for the postcard to be sent, Mr. Olin probably wouldn't have raised that possibility, since it would've just raised Enslin's interest in the room.
If Olin really wanted to keep anyone from ever using the room why didn't he just have the windows and door carefully covered up? Heck, if he really wanted to keep Enslin out why didn't he simply claim that the room was undergoing renovations? It's not as though the law states that the hotel has to provide that specific room on demand.
Actually it does. This is made a big point of in both story and movie. (The logic remains for sealing it off. The renovations answer would quickly break down for someone as determined as Mike, though.)
In the story, Olin explains that the owners don't believe the room is evil and would fire him if he tried to seal off the room.
The story also explains that due to civil rights laws that prevent things like the managers putting minority clients into crappy rooms, anyone who demands a specific room at a hotel can get it, as long as it's not occupied. Normally, Olin doesn't let guests into 1408, but since Enslin insisted on that specific room...
Possible Fridge Brilliance: What if they tried that, and it didn't work? The room can clearly warp reality within itself, so it's not unrealistic for it to have enough control in the general vicinity to prevent its supply of victims from being cut off entirely.
Why bother to clean the room? That just puts the cleaning staff in danger for no reason.
They're required by law to clean every room after a certain amount of time, whether it's been occupied or not. As noted above, the room has to be provided if demanded, and it can't legally be occupied if not cleaned. To his credit, at least Olin employed specific safety measures in having the room cleaned, and while declining to do so is the logical choice, it's not the practical one.