set hint.agp.0.disabled=1

The device.hints file is read in by the boot loader(8) when the system is
about to start, and its contents are passed to the kernel. It contains
various variables to control the boot behavior of the kernel. These
variables are typically “device hints”, but can include any kernel tun-
able values.

The file contains one variable per line. Lines starting with the `#’
character are comments and are ignored by the boot loader.

After the file is read by the boot loader, you may examine the variables
with the show command, and may add a new variable, modify an existing
one, or delete a variable with the set and unset commands of the boot
loader (see loader(8)).

After the system has started, you can dump these variables with the
kenv(1) command.

The numbers (agp.0 , agp.1 agp.2) are device instances and in case of the agp(4) driver there usually is just one instance but in rare case, if you have very old hardware you may have more than one AGP controller in a system, and you need disabling a device completely in case the driver causes problems.

In prompt command (pfsense):
set hint.agp.0.disabled=1
boot-conf

permanent add to /boot/loader.conf
hint.agp.0.disabled=1