It would help if you cite the rules in question. In general, when the attacker has a rule which forbids damage reduction the target suffers full damage.
IGNORING WOUNDS VS RULES THAT PREVENT MODELS FROM IGNORING WOUNDS
Some models have a rule that says that they cannot lose more than a specified number of wounds in the same phase/turn/battle round, and that any wounds that would be lost after that point are not lost. When such a model is attacked by a weapon or model with a rule that says that enemy models cannot use rules to ignore the wounds it loses, that rule takes precedence over the previous rule, and if that attack inflicts any damage on that model, it loses a number of wounds equal to the Damage characteristic of that attack, even if it has already lost the specified number of wounds already this phase/turn/battle round.