How can I completely disable NAT?

From PFSenseDocs

To completely disable NAT to have a routing-only firewall, do the following.

1. Go to the Firewall -> NAT screen, and click the Outbound tab.
2. Select the option "Manual Outbound NAT rule generation (Advanced Outbound NAT (AON))" and click Save.
3. Remove all automatically generated NAT rules at the bottom of the screen.
4. Apply changes

You can NAT on some interfaces and not others by configuring your Outbound NAT rules accordingly.