Do you allow calling outside of the NANPA? These high cost toll numbers are more than just 1-900...
Can't speak for anyone else, but I've got international calling turned off at my provider (it's just a home system).
If I ever need to make an international call I can temporarily enable it. Hasn't happened in nearly a decade of using VOIP.
Another (home) advantage is using a provider that pulls from an existing credit rather than billing as you make calls. The most I'll ever get stung for is $30 since I add $25 any time my balance drops to $5.
I realize it's not in the telco's monetary interest, but you would think somebody would get wise and offer service that includes some intelligent cut-offs (lots of calls after hours or on weekends, calls to 900 or other odd destinations, etc). It won't eliminate the fraud, but it might help.