Actually they most certainly can and have. The ERC20 contract for USDT has a blacklist function and the administrator can block transfers for arbitrary addresses, rendering it worthless. Same with USDC.

They can mint a quadrillion Tether today and that would do the job pretty well. The Tether would still be in the wallets, but at that point it would be quicker to just read from /dev/urandom if you wanted some random-ish bits.