How can I turn Limewire off as my default for downloading video torrents?

Same way you change defaults for most programs. Highlight one of the Torrent files you download from a website, then right-click, choose open with & tick always open with, so you can choose another program instead. You may have a problem with a torrent you've already started downloading in LW though. I'd suggest removing it from LW's download list before you change default program.

BTW some browsers may have a default for how files are handled as well.