Click on The Elder Scrolls Online (PTS) after it's done updating. Click Game Options. Select Uninstall.
(Make sure The Elder Scrolls Online (PTS) is the one Highlighted before you select Uninstall)
Launcher=>Settings(Gear Icon on the Right)=>Options=>Show Public Test Environment(Uncheck)
You may be able to cancel the download by selecting it while it is updating, Selecting settings under Game Options, Turing off Auto-Update, Selecting Game Settings again, and clicking Cancel.