Currently it does not check the difficulty. I have been thinking of how to approach that. I really don't want to make a new set of cvars for each difficulty, but I have to do something.
For expert, I am thinking of either disabling the limit (allowing l4d_ff_limit -1 and 0) or making a new limit for expert only that will ban after X incaps or player kills.
And for advanced I will either have to add new limits, or make a multiplier. I am thinking that a 5X multiplier would work fine for l4d_ff_limit and l4d_ff_teammate (and leave grenade limits alone), but this needs testing. If it works well enough I might not make this a cvar and leave it as a built-in feature.
Also I have finished a permban after X offenses within Y days, but this needs testing before release.