If it's sent by server as a skill (I didn't check that), I think it could be done by hooking these methods in UGameEngine class in engine.dll:
void __thiscall UGameEngine::OnReceiveMagicSkillUse(User*, User*, L2ParamStack&)
void __thiscall UGameEngine::OnReceiveMagicSkillUse(User*, int, L2ParamStack&)
Just get skill ID from param stack, compare it to skill ID you want to ignore and if it matches, just don't call the original function.