I can't make anything of this sentence. Wouldn't miners be the only ones to "freeze" assets on an Ethereum address, and only if 51% of them collude to not accept blocks in which funds from those addresses are moved?
How would Tether "freeze" one of the tokens, so it cannot be moved anymore?
Here is the Tether USDT contract https://etherscan.io/token/0xdac17f958d2ee523a2206206994597c....
There is a map `isBlackListed: address => bool` that stores blacklisted accounts. Then the transfer function starts with
function transfer(address _to, uint _value) public whenNotPaused {
require(!isBlackListed[msg.sender]);
...
}