About the only way to hard brick your Tab is by damaging the Primary boot loader (boot.bin) and the Secondary boot loader (sbl.bin). If these are damaged, you can't get to download or recovery mode anymore, and need extra hardware to be able to flash a ROM again. You can do this for instance by yanking out the cable during a flash, but even then you would have to be unlucky, because these files are small, and will be flashed in seconds. Flashing a ROM with boot loaders for the wrong Tab could do it too I bet.
BTW, it sounds like you are trying to hard brick it, and are frustrated you're not succeeding. Surely I'm wrong about that.