I don't see an obstacle to such an attacker, for example, zeroing their debt with all their creditors, or causing somebody to trust them and owe them money (although there's no reason to believe that person would pay that debt), or transferring arbitrarily large amounts of XRP from other accounts, or even creating XRP (although if they broke the 100 billion XRP cap, I don't know if validators would consider it valid). As far as I can tell, they can rewrite the ledger at will, so long as it appears valid afterwards.