To identify the type of hub, see
SheldonIf it's a threaded hub that takes a separate screw-on freewheel, just screw on a fixed sprocket instead, and don't rely on leg braking. You can use an old-style bottom bracket lockring to make it harder for the cog to unscrew itself.
If it's a freehub (aka a cassette hub) that takes a slide-on cassette, it's either a Surley Fixxer, or try assorted bodges to jam up the freewheeling part of the freehub body - fill with araldite, braze the inner and outer sections together, or whatever.