As it appears that a ball contacting the ODB must be ruled either a blocked ball or interference, what is to stop the defense from using this to their advantage to kill a play and return runners to the last base touched or attempt to get a free out?

The umpires.

Use the same judgment and authority that allows you to refuse to rule obstruction when you judge a runner goes out of their normal running pattern to appear to be impeded, to refuse to award a base to a batter that appears to want to be hit by the pitch, and to declare a catcher aiming at a runner out of the running lane without making an attempt to make the play an unsportsmanlike act. Rule 10.

In addition to the information from AtlUmpSteve.

In most cases the defense would be taking quite a chance trying to force that blocked ball.:rolleyes: