" Is it safe and effective to spray soapy water on them? "

It maybe relatively 'safe' but not the slightest bit 'effective'!! Soapy water - or more properly, insecticidal soap - targets soft bodied, sucking insects, which black flies are not. At best it might annoy them :-)

Are you certain of your identification? Females are blood suckers and males feed on nectar and November is very late anywhere in North America for black flies to be active. Not a huge number

of nectar sources available now. Their adult activity peaks in mid to late summer and by now they are gone for the season. About the only thing associated with black flies at this time of year are their pupae, which require flowing water.

At most, they are a temporary annoyance.