I posted about the Gusset Single Speed Converstion Kit awhile back but it has dropped a little more in price and is still pretty much the cheapest and easiest way to convert your geared bike to a 1×1. Here’s the JensonUSA description:

Go singlespeed the easy way with this kit from Gusset! Why bother with expensive, dedicated singlespeed hubs? With this kit you can use any standard Shimano® 8/9 speed compatible freehub.

Simply slide on cog, sandwiched betweent the two spacers and tighten it down with the lockring. The spacers position can be reversed to adjust the chainline.

Kit includes 16 and 18 tooth cogs, 14 and 19mm spacers, and a lockring.