I'd go with a completely new rear wheel. I'm assuming your rear wheel currently has a freewheel-threaded hub with a freewheel cassette on it (a bunch of small cogs all on the rear hub?) and wha the bike shop did was just remove the gearing mechanism and leave it "permanent" on one gear ratio?

Either way, if you want just a standard single speed freewheel, buy a new backwheel. It will probably be cheaper and easier than buying and installing (or paying someone to install) a new hub on your existing wheel.