Website admin is at http://ip.address.of.switchvox/admin
Under the menus:
System Setup -> VoIP Providers, under "Manage VoIP Providers" in "Add New" select "SIP provider" and click 'GO" button

Click on "Click to Show Advanced Options" to see the full setup, especially for DID's/inbound.
Section "Outgoing Call Rules" is what you want to allow on this trunk/provider. SwitchVox comes with a default set of rules (International, 1-900,toll-free,Long Distance, Local, 911, and Internal). For example, I disabled 1-900 and 911 on this line.

When complete, click "Modify SIP Provider" to save.