I have found the winning submission, can I end my contest early?

Yes. If you have already found the winning submission, you can end the contest early by selecting that entry as winner. In order to select a winner,  on your Entries tab, go to the Entry that you would like to select as winner. In the Actions drop down, click on the Select Winner option. You will be asked for a confirmation, and once you confirm, the winner will be immediately selected.