If it's a plasterboard backbox (i.e plastic) then just replace the whole box, if you have tiled over the face meaning that you can't replace it then replace the tap, push the tap all the way to the back then pull inwards (you might need a screwdriver to pop it out) if none of the above work then rethread it with the tooll above
are you sure it's the box that has threaded and not the screw? also the little brass nuts in the tab can sometimes come loose and spin in the tab meaning you have no option but to replace
If it is spining or threaded you can sometimes angle the screw so it grabs and will tighten but sadly some poor sod will never be able to undo it after you:mad2:
Thank's guys as expected you came up trumps bought the tool ran it through but still only got so far ( brass insert was spinning ), feeling brave I popped out the tab as per colours instructions used the tool from the other side and bob's yer auntie the screw ran through no problem magic wee tool, job done money in the bank and a happy punter. Thanks again guys