Use a blower with a sealed / induction motor (most of them are, in fact) and there's no reason for there to be any sparks. If there were a spark, having extraction means that, at worst, you get a bit of an exhaust flame, because the bulk flow takes any ignition outside. For extra safety, fit a mesh filter shortly before the blower, and that will act as a flame block. In reality, though, I rather suspect that if the extraction is doing its job and keeping the spray booth atmosphere breathable, then the vapours are not sufficiently saturated to ignite.