Not much. happens to the guy in the door
As spells only do what they say they do, what happens when you try to slam the door shut with someone standing in them? The door does not shut. Will it be painful for them? Maybe. Will it do any damage? No, the spell would say so. Will it smack into their face? Depends on where they stand – I would presume that after kicking the door open, they are well on the way through that door. Would it have any mechanical effect if it did? No, see above. (I assume the door opens into the room; in any case the it may also be destroyed by the aforementioned kicking in which case there is not much to use the spell on.)
Obviously, this is a rather strict, RAW reading. Your DM may rule otherwise, allowing for fun consequences.