We've checked and there isn't a way to just turn the faxing option off.
While Snappy Fax is a very good product, have you thought of switching over to a true SMTP service? It's faster, easier to use, and requires no software installation. It might also be considerably cheaper in the long run (no dedicated line rental and possibly cheaper per call charges), depending on what you fax.
Many CAPITAL users have reported that they like this service:
We are also aware of this service, although we have not tried it yet: