A 7spd freewheel, or a freehub?
If it's a freewheel, then singlespeed freewheels are very common. And you won't need any spacers. All you may want to consider is a chain tensioner, unless your frame already has SS slots in the rear.
http://aebike.com/page.cfm?PageID=30...ils&sku=FW1454
