Not unusual to me that vendor still has stuff in the house ... but this is usually agreed before closing, just to facilitate everybody's needs.
Happened to few friends of mine.. Vendor left some stuff in one room for few days .. gentlemen's agreement
I hope the vendor is still not sleeping on the house though
If this is really unbearable to you at this time, throw everything of their stuff on the pavement and changes the locks.... but consider what you are doing, they know where you live!!!
Also, consider that next time, you may be the one who need a "bit of help" to facilitate your move.
I think you are may be over reacting, understandable as you are FTB (in my case, i over reacted for minor things the first time i bought, so i am not different to others)...... moving houses should be an "enjoyable" time, but the complete process is stressful as you never have any guarantee that the deal will really close until the last second...
Talk to the vendor to understand their side of the story.....