+1 on plastic sheeting. I've sealed my area in plastic sheeting with pest control in mind, but also a preventative measure against PM, Botrytis, etc. Depending on what part of the country u r in, seasonal weather, the state of the basement, etc, you might find it easier to control humidity if the grow area is sealed off.
Another thing to think about is the texture of the surrounding surfaces. If this area is something like a poured slab / foundation, and relatively smooth, you might be able to do without the plastic. Generally speaking porous/rough surfaces have many hiding places for bacteria/fungi/spores and can be hard to sanitize.
Basically I would say if there is any carpet, wood (especially wet), damp drywall, wallpaper, etc etc then frame it and seal it with heavy mil plastic sheeting.
Otherwise, scrub the crap out of all surfaces & physically remove as much "gunk" as you can. Then sterilize all the surfaces, post-cleaning. Personally, Id recommend you use something that has a residual disinfecting action. For instance, 30% hydrogen Peroxide will sterilize pretty damn well, but it also breaks down into water relatively quickly and eventually gives the bad nasties an opportunity to re-infect the area. I use a strong mix of some stuff called Physan 20 which is a relatively cheap harsh chemical (Virucide, bacteriacide, fungicide, general biocide) which has some residual effect. There are a number of different cleaning agents out there and I don't think there is one "silver bullet", just get an idea of the different types and methods of operation.
