How to clean smell from (ugh!) non-box ****/pee area (4ft x 8ft)?

ok -- have been keeping cat locked into (large) room
with adjoining bathroom, where cat-box is.

While locked up, and *only* while locked up, he
does indeed use the cat box.

Maybe 4 days thus far.

QUESTION: how to clean-smells from the "illegal" place
he had, maybe three weeks ago, start using. (Really
stinks, from up close).

Advice received here said we had to remove *all* smell
from that area before letting cat get near again.

Methods?

(1) baking power sprinkled over that 4x8 area.

(and then vacuum)

(2) Sprinkle (lots of?) "Nature's Cure" (Is that the
name -- Natures something) over the area.

QUESTION: where to (not so expensively) get LOTS of
that stuff, like by the gallon?

Right now, we get it from the nearby grocery store,
which, only from time to time, will have maybe
ONE bottle (pint?) of the stuff, for three or five
dollars or something.

Gets used up *fast* -- need source of large quantity
of it *or* something like it.

Thanks!

David