I have no experience neither with Slackware nor with ATI hardware, but let's see...
If it is a kernel pre-compiled by your distributor, then check whether AGP support is actually a module, or built-in into the kernel.
You can do that by looking in /proc/config.gz (if available with your kernel) or checking the output of 'lsmod' (type 'lsmod | grep agp')
If it is built-in, then you might give your boot manager (grub or lilo) the kernel boot parameter iommu=noagp (I hope I'm not telling bullshit now; I'm sure there is an noagp option for Knoppix...)
If AGP support is realized via modules, then there are two modules in 2.6.x:
* one generic named 'agpgart'
* one specific to you mainboard chipset! (mine is a VIA chipset, so the module is called via_agp)
To prevent your distro from autoloading it at boot time, there should be a 'blacklist' file in /etc
Look for it, put both module names in there, and reboot.
In Debian, it is /etc/modprobe.d/blacklist
These modules would be installed in /lib/modules/2.6.19/kernel/drivers/char/agp
You could go "brute force" and move these out of the way. It won't break your system.
I guess, it is also possible to put modules inside an initramdisk. I have no experience with that.
Or is it a custom kernel compiled by yourself?
Then 'make menuconfig' and deactivate agp there.
-> Character Devices
< > /dev/agpgart support
Hope that helps