For a scammer tag, the accused person needs to have promised to do something and then failed to deliver on the promise. TradeFortress never promised to pay anyone any bitcoins here. If you trust him to do something that he didn't promise, that's your problem.
An IOU is a promise to pay. TradeFortress has issued IOUs which are essentially promises. He did this using the Ripple system. All of his transactions are part of the ledger. But he hasn't let anyone redeem his promises into actual Bitcoins.
This is no different than someone coming to the forum and saying "I owe you XXX BTC and I promise to pay you in the future." Except that he did it using a cryptographically secure accounting system (called Ripple).
To make matters worse, he took advantage of people by using the forum to promise bitcoins to people in order to get them to extend trust to him in the Ripple system. Then he used that trust to loot their Bitstamp BTC IOUs, replacing them with his worthless "TradeFortress BTC IOUs" (promises to pay Bitcoins).
All of this information is right there in the Ripple ledger. His Ripple address is rH3bZsvVUhzugvcYuJVoSYCEMHkfK6wHNv and you can see his transaction history here:
https://ripple.com/graph/#r3kmLJN5D28dHuH8vZNUZpMC43pEHpaocVThis shows that he has promised users 1,454.65 worth of Bitcoins. But when you try to redeem these promises, he does not pay the Bitcoins he promised.
If the SCAMMER tag applies to someone who makes a promise and then does not deliver, then TradeFortress is the poster child.