If your offer has been accepted by the lender, and if you do have a closing date, then by law, as soon as escrow closes, the property is yours.
You should also exercise your right for a buyer walk through at least 5 days before close of escrow, and maybe even the day before close of escrow to make sure that the seller is vacating the premises.
Unless you and the seller have an agreement where the seller can stay after close of escrow, and you charge him a pre-set rent for each day that he occupies the place (for example, the daily cost of your own PITI, principal, interest, taxes and insurance), then the seller has no rights to stay. If the seller doesn't leave, you have every right to evict the seller and change the locks (which is something you should do anyway upon buying a previously owned house).
Share your concerns with your realtor who will guide you through the process.