Yes, you can end your listing early, but beware.
If your item has bids, ending your listing early means selling your item to the highest bidder. Only if your item has no bids, it is not sold. Listing fees you have paid still apply if you end your listing early.
To End a listing early (immediately) and sell to the highest current bidder, go to: My Account -> Listings -> Active, and select the listing you want to end early. Scroll to the bottom of the page and click the End Early' button.