A screen by any other name.

[MechanicGuy]

Then clearly they haven't been given time and space

Why would they need to give the defender this? Doesn't sound like you're describing a "blind screen" at all....

As far how much movement you allow in a post-up vs a screen in this situation, in application, it depends on whether the defender being screen is attempting to guard the screener/poster-upper or if they're trying to switch to another player and being prevented from doing so. In the latter play, I would allow much less movement or jostling.