well, the simplest way to make that a single speed, would be to remove the derailers and the shifters (non-destructivley, of course), and shorten the chain to fit around one of the existing cogs and one of the chainrings. the other cogs and ring will still be there doing nothing, but you'll have a fine cruising bike.
and you'll easily be able to change it back to a geared bike in the future.

minimal cost (you may have to buy a chainbreak tool...those are around $15 or so) and quick work.

good luck!