a spring wrap up or a fixed tensioning pulley for chain tension
will let you convert to a singlespeed freewheel bike.

If you spring for a us made eccentric axle hub, then there is a possibility of a fixie or a single speed.
on a vertical dropout frame.

whole section on that stuff.. http://www.bikeforums.net/forumdisplay.php/178-Singlespeed-amp-Fixed-Gear
it's so trendy, it's a crowded marketplace.
you can just get one finished and brand new
for the cost of the conversion parts, or less.

me: I need more than 1 gear ratio..
My Internal gear hub bikes, get more riding than the derailleur bikes..