assuming the hardrock has vertical dropouts your only problem is going to be achieving correct tension. once you've removed the derailleurs and the chainrings you don't need, you'll need to do away with the cassette. assuming a splined hub, you'll need a bmx cog, which you'll need to space correctly along the length of the hub for a good chainline. you can use cassette spacers or pvc pipe to do this. if at this point you cannot achieve good chain tension you'll need something like a surly singleator chain tensioner, which attaches to the rear derailleur mount. replace your chain with a bmx one, which is designed not to flex along its length, and you're good to go.