How do you stop a paladin from falling?
The answer: Don't. Let them fall (the fall is glorious, is it not? lol).
Create a Blackguard kit. Then create a carbon copy of your kit's 2DA but rename it CLABPA05.2DA and place it in your override directory. When your paladin falls (after committing the first "evil" act), he or she will stop using the Blackguard 2DA and begin using the Fallen Paladin 2DA, which is identical. Slick, huh?
This works because a Fallen Paladin does not cease to be a Paladin. The Fallen Paladin simply starts using the Fallen Paladin 2DA, which contains three spells: remove priest spells button, remove turn undead button, and remove lay on hands (I think those are the three spells anyway, its been awhile since I've scripted the Blackguard of Bane). Your paladin should retain his abilities and continue to gain priest spells at the appropriate levels and whatnot.
I suggest that your Blackguard gain "Doom" in place of "Detect Evil" and gain "Protection from Good" instead of "Protection from Evil". I also suggest giving your Blackguard several uses of a custom "Charm Undead" spell - it is only appropriate. ;-) (I have a custom Blackguard of Bane kit that uses these abilities... and could send it to you if you desire).
Your class will, unfortunately, list as Fallen Paladin on the portrait screen, but your kit description will remain Blackguard. Also note that this method cannot be used to add multiple Blackguard kits (because there is only one CLABPA05 file). Note, too, that a normal paladin will become a Blackguard if he or she falls (the dark gods are eager to embrace and empower those fallen from the light... LOL. I love being evil... its just so damn fun).
I hope this helps.
Dark blessings on you,
a Wounded Lion