Here, Ripple has frozen someone's assets already, so they have practise...good luck when the first terrorist, Iranian bank or darknet portal uses XRP.... 
http://insidebitcoins.com/news/not-so-decentralized-ripple-freezes-1m-in-user-funds/31862
Once the transaction completed, however, Ripple Labs demanded that Bitstamp use its gateway power to freeze McCalebs account and prevent him from withdrawing the actual cash.
Ripple Labs then requested that the transaction be reversed it wanted the $1,038,172 returned
Notice that we had to go to court to do it, just like anyone can. If you have bitcoins at, say, Poloniex, someone can sue Poloniex to have those bitcoins frozen or given to them.http://insidebitcoins.com/news/not-so-decentralized-ripple-freezes-1m-in-user-funds/31862
Once the transaction completed, however, Ripple Labs demanded that Bitstamp use its gateway power to freeze McCalebs account and prevent him from withdrawing the actual cash.
Ripple Labs then requested that the transaction be reversed it wanted the $1,038,172 returned