DWORD m_nModelIndex = 0x0254;
Set it to an invalid value (0 / etc..) You can also do pretty funny stuffs like changing the model of grenades, also it's the way to make an external knifechanger, type sv_precacheinfo into console
Usage:
Mem->Write<int>(LocalPlayer->GetEntityBasePtr() + Netvars->m_nModelIndex, 0);
Hf.