Since the shifters don't work, it already is SS. To clean up the conversion, remove all shifting components, chainrings and the rear cassette. You can use one of your existing chainrings with shorter bolts, and buy a SS conversion kit with a lockring, rear cog, spacers and chain tensioner to replace your cassette and derailleur.
http://www.sheldonbrown.com/singlespeed.html

If you really intend to convert your mountain bike into a touring bike, it won't come cheap.